Add API usage tracking and dynamic task reloading

Features:
- Usage tracking system (usage_tracker.py)
  - Tracks input/output tokens per API call
  - Calculates costs with support for cache pricing
  - Stores data in usage_data.json (gitignored)
  - Integrated into llm_interface.py

- Dynamic task scheduler reloading
  - Auto-detects YAML changes every 60s
  - No restart needed for new tasks
  - reload_tasks() method for manual refresh

- Example cost tracking scheduled task
  - Daily API usage report
  - Budget tracking ($5/month target)
  - Disabled by default in scheduled_tasks.yaml

Improvements:
- Fixed tool_use/tool_result pair splitting bug (CRITICAL)
- Added thread safety to agent.chat()
- Fixed N+1 query problem in hybrid search
- Optimized database batch queries
- Added conversation history pruning (50 messages max)

Updated .gitignore:
- Exclude user profiles (memory_workspace/users/*.md)
- Exclude usage data (usage_data.json)
- Exclude vector index (vectors.usearch)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

adapters/slack/adapter.py

@@ -39,6 +39,7 @@ class SlackAdapter(BaseAdapter):
         super().__init__(config)
         self.app: Optional[AsyncApp] = None
         self.handler: Optional[AsyncSocketModeHandler] = None
+        self._username_cache: Dict[str, str] = {}  # user_id -> username
 
     @property
     def platform_name(self) -> str:
@@ -255,7 +256,15 @@ class SlackAdapter(BaseAdapter):
             }
 
     async def _get_username(self, user_id: str) -> str:
-        """Get username from user ID."""
+        """Get username from user ID, with caching to avoid excessive API calls.
+
+        Sanitizes the returned username to contain only alphanumeric,
+        hyphens, and underscores (matching memory_system validation rules).
+        """
+        # Check cache first
+        if user_id in self._username_cache:
+            return self._username_cache[user_id]
+
         if not self.app:
             return user_id
 
@@ -263,13 +272,20 @@ class SlackAdapter(BaseAdapter):
             result = await self.app.client.users_info(user=user_id)
             user = result["user"]
             profile = user.get("profile", {})
-            return (
+            raw_username = (
                 profile.get("display_name")
                 or profile.get("real_name")
                 or user.get("name")
                 or user_id
             )
+            # Sanitize: replace spaces/special chars with underscores
+            sanitized = "".join(
+                c if c.isalnum() or c in "-_" else "_" for c in raw_username
+            )
+            self._username_cache[user_id] = sanitized
+            return sanitized
         except SlackApiError:
+            self._username_cache[user_id] = user_id
             return user_id
 
     @staticmethod