fix(plugin-loader): address CodeQL path expression and I/O error handling

- Add explicit relative_to() containment check after path resolution so
  CodeQL recognizes the plugin directory boundary (fixes 4 CodeQL alerts:
  Uncontrolled data used in path expression, lines 168/172/189/205)
- Wrap requirements_file.read_bytes() in try/except OSError — on Raspberry
  Pi with flaky SD card storage this can fail; returns False with a clear log
- Wrap marker_path.read_text() in try/except OSError — a corrupted marker
  falls through to a clean reinstall instead of crashing
- Wrap both marker_path.write_text() calls in try/except OSError — pip
  already succeeded at this point so a marker write failure should not
  return False or propagate through the generic exception handler

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/plugin_system/plugin_loader.py

@@ -165,11 +165,31 @@ class PluginLoader:
         if not requirements_file.exists():
             return True  # No dependencies needed
         marker_path = plugin_dir_resolved / ".dependencies_installed"
+
+        # Validate both paths stay within the plugin directory (path containment check)
+        try:
+            requirements_file.relative_to(plugin_dir_resolved)
+            marker_path.relative_to(plugin_dir_resolved)
+        except ValueError:
+            self.logger.error("Dependency paths outside plugin directory for %s", plugin_id)
+            return False
+
+        try:
             current_hash = hashlib.sha256(requirements_file.read_bytes()).hexdigest()
+        except OSError as e:
+            self.logger.error("Failed to read requirements.txt for %s: %s", plugin_id, e)
+            return False
 
         # Skip if requirements.txt hasn't changed since last install
         if marker_path.exists():
+            try:
                 stored_hash = marker_path.read_text(encoding='utf-8').strip()
+            except OSError as e:
+                self.logger.warning(
+                    "Could not read dependency marker for %s (%s), will reinstall dependencies",
+                    plugin_id, e
+                )
+            else:
                 if stored_hash == current_hash:
                     self.logger.debug("Dependencies already installed for %s (requirements unchanged)", plugin_id)
                     return True
@@ -186,8 +206,11 @@ class PluginLoader:
             )
 
             if result.returncode == 0:
+                try:
                     marker_path.write_text(current_hash, encoding='utf-8')
                     ensure_file_permissions(marker_path, get_plugin_file_mode())
+                except OSError as marker_err:
+                    self.logger.debug("Could not write dependency marker for %s: %s", plugin_id, marker_err)
                 self.logger.info("Dependencies installed successfully for %s", plugin_id)
                 return True
             else:
@@ -202,8 +225,11 @@ class PluginLoader:
                         "Assuming they are satisfied: %s",
                         plugin_id, stderr.strip()
                     )
+                    try:
                         marker_path.write_text(current_hash, encoding='utf-8')
                         ensure_file_permissions(marker_path, get_plugin_file_mode())
+                    except OSError as marker_err:
+                        self.logger.debug("Could not write dependency marker for %s: %s", plugin_id, marker_err)
                     return True
                 self.logger.warning(
                     "Dependency installation returned non-zero exit code for %s: %s",