feat: Homebrew-compatible releases + PATH-based skills

[llbbl]

Summary

• Add Homebrew-compatible release artifacts (tarballs + checksums) to the CI release pipeline
• Update all upkeep skills to invoke upkeep from PATH instead of a local ./bin/upkeep path, with brew install documented as the prerequisite
• Clean up .gitignore to track docs/ and ignore the new _/ scratch directory

Changes

CI / Release Pipeline

• .github/workflows/auto-release.yml: Add linux-arm64 matrix target; rename each Unix binary to upkeep before archiving; produce upkeep_<version>_<os>_<arch>.tar.gz tarballs (using amd64/arm64 tokens); generate checksums.txt (SHA-256); upload tarballs, checksums, and Windows .exe as release assets
• package.json: Add build:linux-arm64 script

Skills

• skills/upkeep-audit/SKILL.md: Replace ./bin/upkeep with upkeep (PATH), document brew install llbbl/tap/upkeep as prerequisite, add "upkeep not found on PATH" guard
• skills/upkeep-deps/SKILL.md: Same PATH + brew + guard changes
• skills/upkeep-quality/SKILL.md: Same PATH + brew + guard changes

Housekeeping

• .gitignore: Remove docs/ exclusion (docs are now committed); add _/ as a gitignored private/scratch directory

Test plan

• Trigger a release and verify tarballs appear as upkeep_<version>_<os>_<arch>.tar.gz with amd64/arm64 naming
• Confirm checksums.txt is present in the release assets
• Install via Homebrew tap and confirm upkeep resolves from PATH
• Run each skill (upkeep-audit, upkeep-deps, upkeep-quality) with upkeep on PATH and verify no path errors
• Verify docs/ directory is tracked by git and _/ contents are gitignored

Closes #7
Closes #8