feat: channel plugin architecture with decoupled configs

- Add plugin discovery via Python entry_points (group: nanobot.channels)
- Move 11 channel Config classes from schema.py into their own channel modules
- ChannelsConfig now only keeps send_progress + send_tool_hints (extra=allow)
- Each built-in channel parses dict->Pydantic in __init__, zero internal changes
- All channels implement default_config() for onboard auto-population
- nanobot onboard injects defaults for all discovered channels (built-in + plugins)
- Add nanobot plugins list CLI command
- Add Channel Plugin Guide (docs/CHANNEL_PLUGIN_GUIDE.md)
- Fully backward compatible: existing config.json and sessions work as-is
- 340 tests pass, zero regressions

README.md

@@ -216,7 +216,9 @@ That's it! You have a working AI assistant in 2 minutes.
 
 ## 💬 Chat Apps
 
-Connect nanobot to your favorite chat platform.
+Connect nanobot to your favorite chat platform. Want to build your own? See the [Channel Plugin Guide](.docs/CHANNEL_PLUGIN_GUIDE.md).
+
+> Channel plugin support is available in the `main` branch; not yet published to PyPI.
 
 | Channel | What you need |
 |---------|---------------|
@@ -1370,7 +1372,7 @@ nanobot/
 │   ├── subagent.py #    Background task execution
 │   └── tools/      #    Built-in tools (incl. spawn)
 ├── skills/         # 🎯 Bundled skills (github, weather, tmux...)
-├── channels/       # 📱 Chat channel integrations
+├── channels/       # 📱 Chat channel integrations (supports plugins)
 ├── bus/            # 🚌 Message routing
 ├── cron/           # ⏰ Scheduled tasks
 ├── heartbeat/      # 💓 Proactive wake-up