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

docs/project/commands.md

@@ -16,6 +16,7 @@ Type these as regular messages in any channel or DM. No `/` prefix needed — ju
 | `help` | Show all available commands |
 | `time` | Current server time |
 | `sessions` | Active session count + cleanup stale |
+| `new` / `reset` | Start a fresh conversation (clears session for this channel) |
 | `reload` | Reload config.json and skills |
 | `subagents` | List active background tasks |