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

test_all.py

@@ -97,7 +97,6 @@ required = {
     "click": "click",
     "aiohttp": "aiohttp",
     "apscheduler": "apscheduler",
-    "dotenv": "python-dotenv",
 }
 
 for mod_name, pkg_name in required.items():