Align Developer with GitHub Actions workflow

[fredporter]

Outcome

• makes Developer the single integrated local repository and GitHub status surface
• exposes read-only current-branch PR and Actions state through gh
• updates CI dependencies from retired stabilization branches to main
• adds least-privilege permissions, concurrency cancellation, and job timeouts
• removes the broken SnackMachine self-hosted workflow and standalone /api/github/* mutation API
• documents the canonical Developer/GitHub contract

Verification

• backend: 500 passed, 6 existing aiohttp warnings
• focused GitHub tests: 44 passed
• route audit: 189 unique routes, 0 duplicates
• Ruff: passed
• planning governance: passed
• docs non-regression: passed
• Vue: 12 passed
• production build: passed with existing chunk warnings