feat: Twitter/X integration (read-only free tier)

- New client: src/clients/twitter.ts
- Tools: twitter_search_tweets, twitter_get_user_profile, twitter_get_user_tweets, twitter_create_tweet
- REST endpoints: GET /api/twitter/search, /api/twitter/user, /api/twitter/tweets, POST /api/twitter/tweet
- Multi-account env var: TWITTER_{ACCOUNT}_BEARER_TOKEN
- twitter_create_tweet returns clear error about paid tier requirement

Total tools: 36

.env.example

@@ -74,3 +74,10 @@ DISCORD_DEFAULT_BOT_TOKEN=your-discord-bot-token
 INSTAGRAM_DEFAULT_ACCESS_TOKEN=your-instagram-access-token
 INSTAGRAM_DEFAULT_BUSINESS_ACCOUNT_ID=your-instagram-business-account-id
 # For additional accounts, duplicate with INSTAGRAM_{ACCOUNT}_*
+
+# ── Twitter/X API ────────────────────────────────────────────────────────────
+# Get a Bearer Token from https://developer.twitter.com/en/portal/dashboard
+# Free tier: 500 tweets/month read-only. Paid tiers required for posting.
+# For default account:
+TWITTER_DEFAULT_BEARER_TOKEN=your-twitter-bearer-token
+# For additional accounts, duplicate with TWITTER_{ACCOUNT}_*