feat: harden github mutation failure reporting

[ganesh47]

Summary

• classify git-side mutation failures like push rejection more explicitly
• cover PR update conflict paths in addition to PR create failures
• surface the first blocked GitHub gate reason more aggressively in top-level inspect summaries

Validation

• npm test -- test/deliver.test.ts test/inspect.test.ts
• npm run typecheck
• npm test
• npm run build
• /home/ganesh/projects/cstack/bin/cstack.js inspect 2026-03-16T12-49-00-888Z-intent-what-are-the-gaps-in-this-project-can-you-work-o (in /tmp/sqlite-metadata-proposal)