Add provenance and unsigned fields to the skills lock schema

[samuv]

Summary

Stack 2: 1/8 · next: verifier wrapper (blocks on stacklok/toolhive-core#192)

• First PR of the Sigstore signing/verification stack (RFC THV-0080 Stack 2, tracked in Track: Skills lock file + Sigstore signing (RFC THV-0080) #5899). The lock file gains a typed provenance: block recording the verified signer identity — the trust-on-first-use anchor that later installs, syncs, and upgrades will be checked against — and an unsigned: true flag recording an explicit --allow-unsigned trust decision. The two are mutually exclusive.
• Provenance fields are validated with the same syntactic constraints as resolvedReference (graphic characters only, bounded length): the lock file is hand-editable and these values feed the identity policy future verifications enforce.
• The fields graduate from the Extra inline round-trip map into typed schema under version 1 — exactly the path the forward-compat design (from the Stack-1 review) reserved for them. A dedicated test proves old files carrying a provenance: block parse into the typed field with no key duplication; the existing forward-compat test's fixture moves to a still-unknown field (attestations:).
• InstallOptions gains AllowUnsigned plus internal ProvenanceInfo/SigstoreBundle carriers that the verification step (a later PR in this stack) will populate.

No behavior changes: nothing produces or consumes these fields until the verifier lands. The whole feature remains gated behind TOOLHIVE_SKILLS_LOCK_ENABLED.

Type of change

• New feature (non-breaking change which adds functionality)

Test plan

• Provenance/unsigned round-trip through Save → Load; typed field does not leak into Extra
• Extra-map graduation: a pre-existing lock with a provenance: block loads into the typed field, no duplicate keys on re-save
• Validation matrix: valid provenance, mutual-exclusion with unsigned, required identity/issuer, control-character and length rejection
• task test + task lint-fix green; no swagger drift (API wiring comes with the verification PR)

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 92.59259% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.12%. Comparing base (f1dae94) to head (580459f).

Files with missing lines	Patch %	Lines
pkg/skills/lockfile/validation.go	92.59%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6015      +/-   ##
==========================================
+ Coverage   72.10%   72.12%   +0.01%     
==========================================
  Files         720      720              
  Lines       74745    74772      +27     
==========================================
+ Hits        53898    53927      +29     
+ Misses      16991    16985       -6     
- Partials     3856     3860       +4     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.