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/opencode_runtime.py

@@ -711,6 +711,20 @@ class OpenCodeRuntime:
         """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 both the persistent session mapping and the live session.
+
+        Returns ``True`` if anything was cleared.
+        """
+        had_live = self.close_session(conversation_id)
+        had_persistent = self._sessions.get(conversation_id) is not None
+        if had_persistent:
+            self._sessions.remove(conversation_id)
+        cleared = had_live or had_persistent
+        if cleared:
+            logger.info(f"Session reset: {conversation_id}")
+        return cleared
+
     # -------------------------------------------------------------------
     # CLI Mode: Subprocess execution
     # (mirrors OpenClaw's runCliAgent → runCommandWithTimeout pattern)