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

heartbeat/heartbeat.py

@@ -62,6 +62,11 @@ class HeartbeatRunner:
         if not self._config.enabled:
             return 0
 
+        # Clear previous heartbeat jobs to avoid duplicates on restart
+        removed = self._scheduler.remove_by_channel_type("heartbeat")
+        if removed:
+            logger.info(f"Heartbeat: cleared {removed} stale job(s) from previous run")
+
         self._ensure_heartbeat_file()
         tasks = self._parse_heartbeat_md()