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

requirements.txt

@@ -4,7 +4,6 @@
 # Slack adapter
 slack-bolt>=1.27.0,<2.0.0
 slack-sdk>=3.40.0,<4.0.0
-python-dotenv>=1.2.1,<2.0.0
 
 # OpenCode agent runtime (optional — only needed for SDK mode)
 # Install with: pip install opencode-ai