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/utils/welcomedMentees.ts

@@ -0,0 +1,30 @@
+/**
+ * @copyright NHCarrigan
+ * @license Naomi's Public License
+ * @author Naomi Carrigan
+ */
+
+import { appendFileSync, existsSync, readFileSync } from "node:fs";
+
+const filePath = "data/welcomed.txt";
+
+const loadWelcomedMentees = (): Set<string> => {
+  if (!existsSync(filePath)) {
+    return new Set();
+  }
+  const contents = readFileSync(filePath, "utf-8");
+  return new Set(contents.split("\n").filter(Boolean));
+};
+
+const welcomedMentees = loadWelcomedMentees();
+
+/**
+ * Appends a mentee's ID to the welcomed set and persists it to disk.
+ * @param id - The Discord user ID to record.
+ */
+const addWelcomedMentee = (id: string): void => {
+  welcomedMentees.add(id);
+  appendFileSync(filePath, `${id}\n`);
+};
+
+export { addWelcomedMentee, welcomedMentees };