feat: add X (Twitter) API to growth agent

[la14-1]

Summary

• Adds X (Twitter) as a second discovery platform for the growth agent alongside Reddit
• X API v2 pay-per-use search (~$0.30/day, ~$9/month budget with 3 queries x 20 results)
• Same Slack approval flow: platform-labeled candidate cards with approve/edit/skip buttons
• Reply posting routes to Reddit comment or X tweet reply based on candidate platform

New files

• x-fetch.ts — X API batch fetcher with Bearer token auth, author expansions for bio/follower data
• x-reply.sh — OAuth 1.0a tweet reply script (HMAC-SHA1 signing via bun crypto)

Modified files

• growth.sh — Phase 1b: optional X fetch when X_BEARER_TOKEN is set, merges both platform datasets
• growth-prompt.md — Multi-platform scoring prompt with X-specific qualification signals
• trigger-server.ts — POST /x-reply endpoint for tweet replies
• helpers.ts — platform column in candidates DB (with migration for existing DBs)
• main.ts — Platform-aware candidate cards, reply routing to Reddit or X
• spa.test.ts — Updated test helper with platform field

Env vars needed

Var	Purpose
X_BEARER_TOKEN	X API v2 app-only read access (search)
X_API_KEY	OAuth 1.0a consumer key (for posting replies)
X_API_SECRET	OAuth 1.0a consumer secret
X_ACCESS_TOKEN	OAuth 1.0a access token
X_ACCESS_SECRET	OAuth 1.0a access token secret

Test plan

• bash -n passes on all .sh files
• bun test — 2104 tests pass (0 failures)
• SPA tests — 100 pass (including candidate DB round-trip with platform field)
• Biome lint — 0 errors
• Manual: run growth cycle with X_BEARER_TOKEN set, verify X posts appear in merged output
• Manual: verify Slack card shows "X Growth" header for X candidates
• Manual: approve X candidate, verify tweet reply is posted

🤖 Generated with Claude Code

[la14-1]

This feature request targets files in .claude/skills/setup-agent-team/ (growth agent infrastructure), which is outside the scope of automated refactoring. The new files (x-fetch.ts, x-reply.ts, etc.) and modifications to growth.sh all live in that directory. This needs manual review and implementation by a maintainer.

-- refactor/community-coordinator