ci: guard that every bundled SKILL.md carries metadata.github-* provenance

[devantler]

🤖 Generated by the Daily AI Assistant

Part of #38 (Theme 4 — provenance & supply-chain hardening).

Problem

AGENTS.md is emphatic that bundled skills come only from upstream via gh skill install and are never hand-edited to diverge — the install records the true upstream in each skill's metadata.github-* frontmatter (github-repo, github-path, github-ref, github-tree-sha). But scripts/validate-manifests.sh enforces only structural parity (manifest ↔ filesystem ↔ README table). It does not assert that every bundled SKILL.md actually carries that provenance frontmatter, so a hand-authored or provenance-stripped skill would pass CI today — silently breaking the "sourced from upstream, never divergent" contract.

Proposed direction

Extend scripts/validate-manifests.sh with a provenance-presence guard: for every discovered plugins/*/skills/*/SKILL.md, assert the frontmatter contains a non-empty metadata.github-repo (and ideally github-path/github-ref). Fail with a clear message naming the offending skill if absent. Pin the new check in scripts/validate-manifests.test.sh — PASS a fixture with provenance, FAIL a fixture with it stripped — so a future refactor that weakens the guard fails the self-test (the repo's established discipline).

Acceptance criteria

• validate-manifests.sh fails when any bundled SKILL.md lacks metadata.github-repo provenance.
• validate-manifests.test.sh covers both the PASS (provenance present) and FAIL (stripped) scenarios.
• All currently-bundled skills pass the new guard (or any genuinely-local skill is explicitly exempted with a documented reason).

Size

S — single guard function + self-test, mirroring the existing checks; immediately shippable.