feat: add slash commands and context menu command (#16)

## Summary

This PR adds a suite of slash commands and a context menu command to Amari, along with shared utilities and quality improvements across the board.

### New Commands
- **`/create-issue`** — generates a GitHub issue on a specified repo using AI-drafted content (title, description, acceptance criteria)
- **`/create-task`** — creates a task in Naomi's Leantime instance with an AI-drafted description and configurable priority
- **`/onboard-mentee`** — automates the mentorship onboarding flow (GitHub invite, forum thread, role assignment)
- **Forward to Owner** (context menu, message command) — forwards any message to Naomi with action buttons (contributed by @teklu)

### Shared Utilities
- **`src/utils/makeAiRequest.ts`** — a single wrapper around the Anthropic SDK for all AI calls, with Amari's personality prompt baked in and full error handling
- **`src/events/handleInteractionCreate.ts`** — extracted interaction handler (was inline in `index.ts`) to keep complexity under control

### Quality Improvements
- `ephemeral: true` → `flags: [ MessageFlags.Ephemeral ]` (deprecated API removed)
- Full `try/catch` + `logger.error` audit across all modules (`logMenteeJoin`, `checkAchievements`, `processMentorshipRole`, `processGitHubEvent`)
- `deployGlobal.ts` replaced with a static `commands.json` payload for manual registration
- Amari's personality prompt updated to reflect her actual character — warm, observant, and relentlessly caring

### Notes
- `CLIENT_ID` is needed in 1Password at `op://Environment Variables - Naomi/Amari/client id` for the `commands.json` registration call
- The forward-to-owner command (PR #13, contributed by @teklu) is fully preserved with original commit authorship

✨ This PR was created with help from Hikari~ 🌸

Co-authored-by: Naomi Carrigan <commits@nhcarrigan.com>
Co-authored-by: Teklu <tekluabayneh@gmail.com>
Reviewed-on: #16
Co-authored-by: Hikari <hikari@nhcarrigan.com>
Co-committed-by: Hikari <hikari@nhcarrigan.com>

src/interfaces/amari.ts

@@ -10,6 +10,7 @@ import type { App } from "octokit";
 export interface Amari {
   discord:      Client;
   github:       App["octokit"];
+  githubApp:    App;
   lastRssItems: {
     freeCodeCamp: string | null;
     hackerNews:   string | null;