feat: mentorship improvements and name mention notifications (#14)

## Summary

- **Name/title mention notifications**: Amari now notifies Naomi when a message contains her name, common nicknames (`nomi`, `nao`, `nae`, `naonao`), or honorifics (`goddess`, `queen`, `mistress`, `your/her majesty`, `your/her highness`). Uses the same cooldown logic as mention forwarding.
- **Simplified mentee onboarding**: Replaced the lengthy welcome message with a concise prompt asking the new mentee to ping Naomi with their GitHub username and name.
- **Removed offboard notification**: `logMenteeLeave` now only logs a metric silently — no more "user must be offboarded" messages in the channel.
- **Deduplicated welcome messages**: Welcomed mentee IDs are persisted to `data/welcomed.txt` so the onboarding message is only ever sent once, even if the role is re-assigned.

## Test plan

- [ ] Assign mentorship role to a user and confirm the new onboarding message appears
- [ ] Re-assign the role to the same user and confirm no duplicate message is sent
- [ ] Remove a mentee from the server and confirm no offboard message is posted
- [ ] Send a message containing a matched name/honorific and confirm Naomi receives a DM forwarding it

✨ This PR was created with help from Hikari~ 🌸

Co-authored-by: Naomi Carrigan <commits@nhcarrigan.com>
Reviewed-on: #14
Co-authored-by: Hikari <hikari@nhcarrigan.com>
Co-committed-by: Hikari <hikari@nhcarrigan.com>

src/events/handleMessageCreate.ts

@@ -5,6 +5,7 @@
  */
 
 import { ids } from "../config/ids.js";
+import { notifyNameMention } from "../modules/notifyNameMention.js";
 import { respondToMention } from "../modules/respondToMention.js";
 import { updateMentorshipThread } from "../modules/updateMentorshipThread.js";
 import type { Amari } from "../interfaces/amari.js";
@@ -29,4 +30,5 @@ export const handleMessageCreate = async(
   }
   await updateMentorshipThread(amari, message);
   await respondToMention(amari, message);
+  await notifyNameMention(amari, message);
 };