docs: proposal 60 - automated #buildinpublic Twitter posting from GitHub commits

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What

Adds proposal #60: a fully automated system that watches PlanExeOrg/PlanExe commits, generates a #buildinpublic tweet via LLM, and posts to Twitter with no human in the loop.

Why

Neither Simon nor Mark wants to manually maintain social media for PlanExe. This closes the gap: steady, technical #buildinpublic updates generated from real commits = passive discoverability with zero ongoing human effort.

What's in the proposal

• Concept & architecture: GitHub API polling → LLM generation → Twitter post (bird CLI or Twitter API)
• State file pattern: last_commit_sha.txt to prevent duplicate tweets
• LLM prompt template: technical tone, no hype, auto-include commit URL + #buildinpublic
• Cron schedule options: per-commit / daily digest / weekly summary (daily digest recommended)
• Implementation sketch: pseudocode for shell + Python posting
• Decision table: 8 questions Simon needs to answer before implementation starts
• Suggested starting config: daily digest, Claude Haiku, GitHub Actions cron, dedicated @PlanExeBuilds account

What's NOT included

This is docs-only. No code, no credentials, no working implementation. Just the proposal for Simon to review and decide on.

Decisions needed from Simon

See the proposal's decision table — key ones:

1. Which Twitter account posts?
2. Per-commit, daily digest, or weekly summary?
3. All commits, or PR merges only?

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Docs only — one file added: docs/proposals/60-buildinpublic-twitter-automation.md