chore: Create upstream sync skill

[KemingHe]

Problem

Need documented, repeatable process for syncing upstream git-filter-repo updates from newren/git-filter-repo into local main branch, then selectively promoting to development branch.

Proposed Solution

Create .agents/skills/sync-upstream-main/ with:

SKILL.md - Agent skill file containing:

• When to use: Upstream release detected or scheduled check (3-6 month intervals)
• Process steps:
  1. Fetch upstream remote
  2. Rebase or merge into local main branch
  3. Run upstream tests on main to verify
  4. Identify files needing copy to development (primarily git_filter_repo.py)
  5. Copy and test on development branch
  6. Update CHANGELOG or commit noting upstream version
• Error handling: Breaking API changes, merge conflicts
• Validation: Test both branches independently

README.md - Skill directory overview:

• Purpose of skill
• When to invoke
• Manual workflow if skill unavailable

Parked in design-parked.md as "Revisit when: first upstream update needed or 3-6 months post-launch."

Alternatives Considered

Manual sync documentation in CONTRIBUTING.md, but dedicated skill provides agent-executable workflow and reduces maintainer cognitive load.