feat(ci): enforce skill identity and version rules (3/4)

[miscreantmoogly]

Stacked PR 3 of 4. Based on #127; review and merge in order. This diff shows only this PR's changes.

Description

With --base-ref, each skill is compared with its merge-base copy.

Identity:

• A skill folder can't be removed or renamed; deprecate it instead.
• Emergency removal. For malicious content or a legal or security request, a folder may be deleted when the same PR lists the skill in skill-rules/removed-skills.json, with a reason, the approving maintainer and the date. A listed skill's folder must be gone.
• A path removed earlier in main's first-parent history can't be reused. Today that's eks-operation-review.

Versions:

• A change to any published file needs a higher metadata.version. A published version is immutable.
• The version goes up one step at a time: from 3.4.0 to 3.4.1, 3.5.0 or 4.0.0, never 79.5.0. It never goes down.
• A new skill starts at 1.0.0.
• A bump without a CHANGELOG.md change is a warning.
• A base copy still on a two-part version, such as 2.6, is compared as 2.6.0.

Content hash. SHA-256 over sorted <path>\0<file sha256>\n records of the published files. A golden fixture in conformance-cases.json pins it, so any other implementation can prove it computes the same hash.

Type of change

• Documentation or infrastructure change

Testing

• 130 tests pass. On throwaway repos they show:
  • removal and rename fail, and an emergency removal listed in removed-skills.json passes;
  • reusing a retired path fails, including a folder renamed away on main;
  • a path that only existed on a merged branch is not retired;
  • a content change without a bump fails, and passes once bumped;
  • the comparison uses the merge base, not the base tip;
  • a changed hash function stops the check through the golden fixture.
• Conformance cases cover one-step bumps, skipped and far jumps, a new skill's first version, and emergency removal.
• --base-ref origin/main: 34 skills, 0 errors. The two version fixes from feat(ci): validate skill frontmatter on pull requests (1/4) #126 are single steps.

[ams-thakkar]

Holding this one, and #125 likewise: @aadimch and @miscreantmoogly have each built the same check independently, and the two are not compatible as they stand — most pointedly, metadata.deprecated is specified oppositely (quoted "true" here, unquoted true in #125), and each documents its own form in CONTRIBUTING.md.

I have asked the two of you to agree on which set to keep.