ci: gist-based conflict resolution for v4 → v4-next sync

[AztecBot]

Summary

Replaces the current v4 → v4-next conflict handling (which creates a PR with conflict markers + spams Slack/ClaudeBox on every v4 push) with a gist-based approach:

• On conflict: Creates a GitHub gist with conflict analysis (files, diff, resolution hints) instead of a PR with conflict markers
• Deduplication: Subsequent v4 pushes that still conflict update the same gist — no new Slack messages, no spam
• Human-in-the-loop: Posts to #backports once, human tags @claudebox in the thread to opt-in to auto-resolve
• Auto-cleanup: Deletes the conflict gist when a subsequent merge succeeds (conflicts resolved)
• Prompt .txt files: All text templates (gist summary, Slack message, ClaudeBox prompt) are in .github/prompts/*.txt for easy editing

What changes vs today

	Current	Proposed
On conflict	Creates branch + PR with conflict markers	Creates gist with conflict analysis
CI	Runs on the broken PR (wastes CI)	No CI until resolution PR is created
Spam risk	Every v4 push creates new branch/PR/notification	Gist is idempotent — updated in place
ClaudeBox	Auto-dispatched (spam risk)	Human opts in by tagging in Slack thread
Branch noise	sync/v4-into-v4-next-* branches accumulate	No branches until resolution

Files

• .github/workflows/pull-v4-into-v4-next.yml — rewritten workflow
• .github/prompts/v4-v4next-conflict-summary.txt — gist markdown template
• .github/prompts/v4-v4next-slack-conflict.txt — Slack notification template
• .github/prompts/v4-v4next-claudebox-resolve.txt — ClaudeBox prompt for resolution

Based on design: https://gist.github.com/AztecBot/b44f2a2d3afaada77d1033174c49b6c4