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/cache/disk_cache.py

@@ -68,14 +68,15 @@ class DiskCache:
             return None
         return os.path.join(self.cache_dir, f"{key}.json")
     
-    def get(self, key: str, max_age: int = 300) -> Optional[Dict[str, Any]]:
+    def get(self, key: str, max_age: Optional[int] = 300) -> Optional[Dict[str, Any]]:
         """
         Get data from disk cache.
-        
+
         Args:
             key: Cache key
-            max_age: Maximum age in seconds
-            
+            max_age: Maximum age in seconds; None disables age-based expiry
+                (the record never counts as stale). Mirrors MemoryCache.get.
+
         Returns:
             Cached data or None if not found or expired
         """
@@ -105,7 +106,13 @@ class DiskCache:
                     record_ts = None
             
             now = time.time()
-            if record_ts is None or (now - record_ts) <= max_age:
+            # max_age=None means "never expires" (mirrors MemoryCache and the
+            # cache_manager docstring). Guard it explicitly — otherwise the
+            # comparison below raises TypeError and the record is treated as a
+            # miss, which silently breaks callers that persist long-lived state
+            # via get(key, max_age=None) (e.g. plugin health/metrics that must
+            # survive restarts and be read cross-process).
+            if record_ts is None or max_age is None or (now - record_ts) <= max_age:
                 return record
             else:
                 # Stale on disk; keep file for potential diagnostics but treat as miss