feat(growth): add Phase 0 — daily tweet draft + X mention engagement

[la14-1]

Summary

• Adds Phase 0 to the growth agent, running before Reddit scanning each cycle
• Phase 0a — Tweet Draft (always runs): Reads last 7 days of git commits → Claude drafts a ≤280 char tweet about new features, fixes, or best practices → posts Block Kit card to Slack
• Phase 0b — X Mention Search (credential-gated): x-fetch.ts searches X API v2 for Spawn/OpenRouter mentions → Claude scores and drafts engagement replies → posts card to Slack
• All cards require human approval in #spawn-growth — nothing is ever auto-posted to X
• Phase 0b gracefully skips when X_API_KEY is not set; auto-activates when credentials are added

New files

• tweet-prompt.md — Claude prompt for tweet generation from git history
• x-engage-prompt.md — Claude prompt for scoring X mentions + drafting replies
• x-fetch.ts — X API v2 search client with OAuth 1.0a, valibot schemas, dedup against DB

Modified files

• growth.sh — Phase 0a + 0b before Phase 1, cleanup trap updates for new temp files/PIDs
• helpers.ts — tweets table schema, TweetRow interface + CRUD, logTweetDecision()
• main.ts — TweetPayloadSchema, XEngagePayloadSchema, postTweetCard(), postXEngageCard(), 8 new Slack action handlers (approve/edit/skip for both tweet + engage)

Test plan

• bash -n growth.sh — syntax check passes
• bun build — all 4 TypeScript files compile clean
• Zero as type assertions in all modified files
• Trigger growth cycle → verify Phase 0a runs, drafts tweet card in Slack
• Verify Phase 0b logs "skipping (no X credentials)" gracefully
• Click Approve/Edit/Skip on tweet card → verify DB updates + Slack message replacement
• Add X credentials → verify Phase 0b activates and engage cards appear

Filed from Slack by SPA

🤖 Generated with Claude Code