feat: openclaw-style secrets (env.vars + \) and per-task model routing

- Replace python-dotenv with config.json env.vars block + \ substitution
- Add models section for per-task model routing (heartbeat, subagent, default)
- Heartbeat/subagent tasks can use different models/providers than main chat
- Remove python-dotenv from dependencies
- Update all docs to reflect new config approach
- Reorganize docs into project/ and research/ subdirectories

agent/claude_runtime.py

@@ -206,6 +206,17 @@ class ClaudeCodeRuntime:
         """Clean up stale sessions. Returns count removed."""
         return self._sessions.cleanup(self._config.session_ttl_hours)
 
+    def reset_session(self, conversation_id: str) -> bool:
+        """Clear the persistent session mapping for a conversation.
+
+        Returns ``True`` if a session was cleared.
+        """
+        had = self._sessions.get(conversation_id) is not None
+        if had:
+            self._sessions.remove(conversation_id)
+            logger.info(f"Session reset: {conversation_id}")
+        return had
+
     # -------------------------------------------------------------------
     # Core: Run claude CLI
     # -------------------------------------------------------------------