Add Unix social tools section to dev onboarding doc

Documents who, w, finger, write, wall, ytalk and .plan files.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/dev-onboarding.md

@@ -170,8 +170,32 @@ In the maubot admin UI, go to "Logs" to see recent activity and errors.
 - [Maubot Plugin Examples](https://github.com/maubot)
 - [Matrix Python SDK (mautrix)](https://docs.mau.fi/python/latest/)
 
+## Shared Server Tools
+
+This is a shared Unix server. Classic tools for seeing who's around and communicating:
+
+| Command | What it does |
+|---------|--------------|
+| `who` | See who's logged in |
+| `w` | Who's logged in + what they're doing |
+| `finger <user>` | User info + their `.plan` file |
+| `write <user>` | Send a message to their terminal |
+| `wall` | Broadcast message to all logged-in users |
+| `ytalk <user>` | Split-screen chat (both must be online) |
+
+### Your .plan file
+
+Create `~/.plan` to tell others what you're working on:
+
+```bash
+echo "Building a dice bot. Ask me about random numbers." > ~/.plan
+```
+
+Others can see it with `finger yourname`.
+
 ## Getting Help
 
 - Ask in `#devs-sandbox` - other devs and admins can help
+- Use `write` or `ytalk` if another dev is online
 - Check the maubot logs for error messages
 - Review the maubot documentation