feat(social): implement Twitter/X tools — search, timeline, tweet

[EngineerProjects]

Context

Stub is at `internal/tools/social/twitter/tool.go`. All methods return `ErrNotImplemented`. `IsEnabled() = false`.

Tools to implement

Tool	Description
`twitter_search`	Search recent tweets (last 7 days on free tier)
`twitter_timeline`	Fetch a user's recent tweets
`twitter_tweet`	Post a tweet
`twitter_thread`	Post a connected thread of tweets

Auth + cost reality check

Free tier (2025):

• Bearer token only → read-only search, last 7 days, 500k tweets/month
• Write: 1,500 tweets/month app-wide, 300/user/day
• No user lookup by username without Basic tier

Basic tier ($100/month):

• Full user lookup, 2M tweet reads/month, better rate limits

Recommendation: implement search (bearer token, free) first. Gate write tools behind a `TWITTER_API_KEY` check in `IsEnabled()`.

API

Twitter API v2: `https://api.twitter.com/2/\`

Endpoints:

• `GET /2/tweets/search/recent?query=&max_results=` — recent search (bearer)
• `GET /2/users/:id/tweets` — user timeline (bearer)
• `POST /2/tweets` — create tweet (OAuth 1.0a or OAuth 2.0 user context)

Required env vars:

• `TWITTER_BEARER_TOKEN` — for read tools
• `TWITTER_API_KEY`, `TWITTER_API_SECRET`, `TWITTER_ACCESS_TOKEN`, `TWITTER_ACCESS_SECRET` — for write tools

Implementation notes

• OAuth 1.0a signing for write endpoints — use `golang.org/x/oauth1` or implement HMAC-SHA1 manually
• `twitter_tweet` and `twitter_thread` require `RequiresPermission: true`
• Thread tool: POST each tweet with `reply.in_reply_to_tweet_id` from previous
• Rate limits are strict — add backoff + 429 handling