feat: activate dormant plugin health/metrics subsystem and surface it in the web UI (#388)

* feat(plugin-system): activate dormant plugin health & metrics subsystem

PluginManager shipped a fully-built health tracker, resource monitor and
circuit breaker that were never instantiated (health_tracker/resource_monitor
were left as None), so the circuit breaker never engaged and the existing
health/metrics API routes always returned "not available".

- DisplayController now wires a PluginHealthTracker and PluginResourceMonitor
  onto the plugin manager, enabling the circuit breaker (a repeatedly-failing
  plugin's update() is skipped after consecutive failures, then retried after
  a cooldown) and per-plugin execution-time metrics. Both persist to the
  shared cache.
- load_plugin() now validates each plugin's config against its JSON schema in
  a strictly warn/degrade-only way: a violation logs a warning and flags the
  plugin degraded in the health tracker, but never changes whether the plugin
  loads or its pass/fail behaviour. Adds PluginHealthTracker.set_degraded(),
  which never touches the circuit breaker.
- ResourceMonitor CPU/memory sampling now reuses a cached psutil.Process and
  reads cpu_percent(interval=None), so monitoring no longer blocks ~100ms per
  call on the display loop's update path.
- Fix DiskCache.get() raising TypeError for max_age=None ("never expires"),
  which silently discarded persisted plugin health/metrics on read and thus
  broke cross-process and post-restart surfacing.
- Fix two dead PluginManager helpers that called non-existent tracker methods.

Tests: new test_resource_monitor, test_plugin_health,
test_plugin_manager_schema_soft; extended test_cache_manager and
test_display_controller.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UvTav268UXv44ub9K11LYq

* feat(web-ui): surface plugin health, metrics and load state

With the health/metrics subsystem now active in the display service, expose it
in the web UI (which runs as a separate process from the display loop):

- Wire a health tracker / resource monitor backed by the shared on-disk cache
  into the web process so /api/v3/plugins/health and /plugins/metrics read the
  data the display service persists.
- Build those route responses per installed plugin id (the tracker's in-memory
  view is empty in a fresh web process) so cross-process data is included.
- Add state + error_info to /plugins/installed entries so the UI can show why a
  plugin isn't running instead of just loaded:false.
- Add a "Plugin Health" panel to the Tools page (circuit status, avg/max update
  time, update count, last error) plus PluginAPI.getPluginMetrics().

Tests: route-level tests for the health/metrics endpoints in test_web_api.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UvTav268UXv44ub9K11LYq

* fix(plugin-metrics): refresh cross-process health/metrics reads; type hints

Addresses CodeRabbit review on #388:

- Major: the web process's health/resource trackers cached the first persisted
  read in an in-memory dict (and the CacheManager memory tier held max_age=None
  entries indefinitely), so a long-lived web process showed the first snapshot
  and never reflected the display service's later updates. Add an opt-in
  force_reload path (get_health_summary/get_health_state/_load_health_state and
  get_metrics_summary/get_metrics) that bypasses the in-memory copy and, via a
  new memory_ttl passthrough on CacheManager.get, the cache manager's memory
  tier — so each /plugins/health and /plugins/metrics poll reads fresh persisted
  state. Default behaviour (force_reload=False) is unchanged for the display
  process and existing callers.
- Minor: DiskCache.get type hint is now Optional[int] with the None ("never
  expires") semantics documented, matching MemoryCache.get.

Tests: new force_reload staleness cases in test_plugin_health and
test_resource_monitor.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UvTav268UXv44ub9K11LYq

---------

Co-authored-by: Claude <noreply@anthropic.com>

src/plugin_system/resource_monitor.py

@@ -71,17 +71,32 @@ class PluginResourceMonitor:
         self.cache_manager = cache_manager
         self.enable_monitoring = enable_monitoring and PSUTIL_AVAILABLE
         self.logger = logging.getLogger(__name__)
-        
+
         # Resource metrics per plugin
         self._metrics: Dict[str, ResourceMetrics] = {}
         self._limits: Dict[str, ResourceLimits] = {}
-        
+
         # Thread-local storage for execution tracking
         self._local = threading.local()
-        
+
         # Lock for thread-safe access
         self._lock = threading.Lock()
-        
+
+        # Cache a single psutil.Process handle. Reusing the same handle is what
+        # lets cpu_percent() be read non-blocking (interval=None): psutil returns
+        # the utilisation since the *previous* call on that same object. Creating
+        # a fresh Process() per call would force interval-based sampling that
+        # blocks the caller — unacceptable on the display loop's update path.
+        self._process = None
+        if self.enable_monitoring:
+            try:
+                self._process = psutil.Process()
+                # Prime cpu_percent so the first real measurement returns a
+                # meaningful delta instead of 0.0.
+                self._process.cpu_percent(interval=None)
+            except Exception:  # pragma: no cover - psutil edge cases
+                self._process = None
+
         if not PSUTIL_AVAILABLE and enable_monitoring:
             self.logger.warning(
                 "psutil not available - resource monitoring will be limited to execution time only"
@@ -95,13 +110,21 @@ class PluginResourceMonitor:
         """Get cache key for plugin limits."""
         return f"plugin_limits:{plugin_id}"
     
-    def get_metrics(self, plugin_id: str) -> ResourceMetrics:
-        """Get current metrics for a plugin."""
+    def get_metrics(self, plugin_id: str, force_reload: bool = False) -> ResourceMetrics:
+        """Get current metrics for a plugin.
+
+        ``force_reload=True`` bypasses both the in-memory copy and the cache
+        manager's memory tier so a read-only consumer (e.g. the web process)
+        sees the writer process's latest persisted metrics rather than a stale
+        first snapshot.
+        """
         with self._lock:
-            if plugin_id not in self._metrics:
+            if force_reload or plugin_id not in self._metrics:
                 # Try to load from cache
                 cache_key = self._get_metrics_key(plugin_id)
-                cached = self.cache_manager.get(cache_key, max_age=None)
+                cached = self.cache_manager.get(
+                    cache_key, max_age=None, memory_ttl=0 if force_reload else None
+                )
                 if cached:
                     metrics = ResourceMetrics(**cached)
                 else:
@@ -137,21 +160,24 @@ class PluginResourceMonitor:
     
     def _get_process_memory_mb(self) -> float:
         """Get current process memory usage in MB."""
-        if not self.enable_monitoring:
+        if not self.enable_monitoring or self._process is None:
             return 0.0
         try:
-            process = psutil.Process()
-            return process.memory_info().rss / 1024 / 1024
+            return self._process.memory_info().rss / 1024 / 1024
         except Exception:
             return 0.0
-    
-    def _get_process_cpu_percent(self, interval: float = 0.1) -> float:
-        """Get current process CPU usage percentage."""
-        if not self.enable_monitoring:
+
+    def _get_process_cpu_percent(self) -> float:
+        """Get current process CPU usage percentage (non-blocking).
+
+        Reads cpu_percent(interval=None) against the cached process handle, so
+        it returns immediately with the utilisation observed since the previous
+        call rather than blocking to sample a fresh interval.
+        """
+        if not self.enable_monitoring or self._process is None:
             return 0.0
         try:
-            process = psutil.Process()
-            return process.cpu_percent(interval=interval)
+            return self._process.cpu_percent(interval=None)
         except Exception:
             return 0.0
     
@@ -281,9 +307,13 @@ class PluginResourceMonitor:
             self.logger.error(error_msg)
             raise ResourceLimitExceeded(error_msg)
     
-    def get_metrics_summary(self, plugin_id: str) -> Dict[str, Any]:
-        """Get metrics summary for a plugin."""
-        metrics = self.get_metrics(plugin_id)
+    def get_metrics_summary(self, plugin_id: str, force_reload: bool = False) -> Dict[str, Any]:
+        """Get metrics summary for a plugin.
+
+        ``force_reload=True`` refreshes from the persisted cache first so
+        cross-process readers reflect the writer's latest metrics.
+        """
+        metrics = self.get_metrics(plugin_id, force_reload=force_reload)
         limits = self.get_limits(plugin_id)
         
         avg_execution_time = 0.0