feat(git-tidy): add content-aware cleanup guidance

[jongio]

Description

Adds content-aware Git cleanup guidance that helps developers decide what to remove, keep, resume, update, open as a pull request, merge, or defer.

Changes

• Inventories branches, worktrees, stashes, remote refs, tags, recovery artifacts, large blobs, and maintenance state.
• Groups equivalent work using exact object and change-unit evidence.
• Presents a numbered Git Clean style report with removal targets, retained copies, reasons, and activity context.
• Requires selection, fresh revalidation, exact command preview, explicit approval, and a second revalidation before mutation.
• Adds typed closed-schema results, bounded content review, trusted Git and GitHub process boundaries, and fail-closed coverage gaps.
• Adds cross-platform CI coverage, strict Vally evaluation scenarios, specifications, and user documentation.

Testing

• npm run test:coverage --prefix skills/git-tidy: 257 passed, 96.23% lines, 91.03% branches, 98.31% functions.
• node --test test/*.test.mjs: 20 passed.
• npm run eval:lint --prefix skills/git-tidy: passed.
• npm run build --prefix site: 13 pages built.
• Production dependency audits: zero vulnerabilities.