Fix 15 remaining CodeQL path-injection and stack-trace-exposure alerts

Switch from resolve()+relative_to() to os.path.basename() reassignment,
which CodeQL recognizes as a path sanitizer that breaks the taint chain.
Also remove exception objects from backup_manager validate_backup return
strings to eliminate the stack-trace-exposure taint source.

Fixes alerts #227, #233, #234, #235, #237, #238, #239, #240, #241,
#242, #243, #244, #245, #246, #247.

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

src/plugin_system/plugin_loader.py

@@ -8,6 +8,7 @@ Extracted from PluginManager to improve separation of concerns.
 import json
 import importlib
 import importlib.util
+import os
 import sys
 import subprocess
 import threading
@@ -68,6 +69,11 @@ class PluginLoader:
         Returns:
             Path to plugin directory or None if not found
         """
+        # Sanitize plugin_id — os.path.basename is a CodeQL-recognized path sanitizer
+        plugin_id = os.path.basename(plugin_id or '')
+        if not plugin_id:
+            return None
+
         # Strategy 1: Use mapping from discovery
         if plugin_directories and plugin_id in plugin_directories:
             plugin_dir = plugin_directories[plugin_id]
@@ -145,6 +151,9 @@ class PluginLoader:
         Returns:
             True if dependencies installed or not needed, False on error
         """
+        plugin_id = os.path.basename(plugin_id or '')
+        if not plugin_id:
+            return False
         # Resolve and validate plugin_dir before constructing any derived paths
         try:
             plugin_dir_resolved = plugin_dir.resolve(strict=True)
@@ -371,6 +380,9 @@ class PluginLoader:
         Returns:
             Loaded module or None on error
         """
+        plugin_id = os.path.basename(plugin_id or '')
+        if not plugin_id:
+            raise PluginError("Invalid plugin ID")
         try:
             plugin_dir_resolved = plugin_dir.resolve(strict=True)
         except OSError: