feat: add sync-forks skill for community fork maintainers

[taslim]

Type of Change

• Skill - adds a new skill in .claude/skills/
• Fix - bug fix or security fix to source code
• Simplification - reduces or simplifies source code

Description

Fork maintainers who customize NanoClaw (adding channels, personas, integrations) need a safe way to push improvements back to their public fork without leaking private data, and pull community contributions back. Today this is a manual, error-prone process.

/sync-forks is a bidirectional fork sync skill with a privacy-first design:

Push flow (private → public fork):

• Optionally runs /update-nanoclaw first to sync your private fork with upstream before pushing — so the public fork gets both upstream updates and your improvements in one go
• Creates a disposable staging branch from the public fork — never modifies your main
• Merges upstream commits individually (preserves community attribution)
• Squashes private fork commits into one (no private history leaks)
• Privacy manifest: declarative config where users list their identifiers, private files, and replacement rules once. The skill auto-restores private files to public fork templates, then grep-scans the entire staged diff as a hard gate — push won't proceed until clean
• Validates build, then opens a PR or pushes directly

Pull flow (public fork → private):

• Creates backup branch + tag before merging
• Merges or cherry-picks selected commits
• Validates build
• Provides rollback instructions (git reset --hard <backup-tag>)

Fork discovery: auto-detects public fork candidates from git remotes (anything that isn't origin or upstream). One fork auto-selects; multiple forks prompts a choice. Multi-fork pull supported.

The privacy manifest is a template — users fill in their own identifiers before first use. The skill file in this PR contains only placeholder examples.