Add project-level skills lock file schema package

[samuv]

Summary

• Why: RFC THV-0080 (merged) proposes a project-level toolhive.lock.yaml that pins the exact content of every project-scoped skill install, giving skills the reproducibility guarantee package-lock.json/Cargo.lock/go.sum provide elsewhere. This is the first PR of a stack productionizing the validated POC in Add project-level skills lock file (POC) #5715 into reviewable, independently-tested changes.
• What: Adds pkg/skills/lockfile, a new leaf package with no callers yet: the lock file schema (Entry, Lockfile), a deterministic content-hash algorithm (ContentDigest) used for on-disk integrity verification, and load/save/upsert/remove operations. All filesystem access is confined to the validated project root via os.Root (OS-enforced containment), not string path validation alone — this avoids the CodeQL "uncontrolled data used in path expression" findings that were flagged on the POC.

Part of the production stack for #5715 (RFC THV-0080). Stack: 1/6 — lockfile → lock-service → install-hooks → sync → upgrade → cli-exitcodes. This PR has no consuming callers; wiring into skillsvc.Install lands in PR3 of the stack.

Type of change

• New feature

Test plan

• Unit tests (task test)
• Linting (task lint-fix)

Table-driven tests cover round-trip load/save, unknown-version hard error, requiredBy cross-reference validation, golden contentDigest vectors (freezing the algorithm), a goroutine-hammer concurrency test against UpsertEntry, and traversal/escape rejection. 89.9% statement coverage.

Does this introduce a user-facing change?

No — this package has no callers yet.

Special notes for reviewers

• Entry deliberately omits provenance/unsigned (Sigstore fields) — those land in PR7 of the Sigstore stack, once Stack 1 (this stack) merges.
• The whole feature (this PR through the final Sigstore PR) stays inert on main behind an env-var gate introduced in PR3, per the incremental-rollout plan — see that PR's description.
• Golden vectors in contentdigest_test.go freeze the dirhash algorithm; treat any future change to it as a lock-file format break, not a routine test update.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 83.90244% with 33 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.61%. Comparing base (fbf2f6d) to head (4997552).

Files with missing lines	Patch %	Lines
pkg/skills/lockfile/lockfile.go	78.70%	12 Missing and 11 partials ⚠️
pkg/skills/lockfile/contentdigest.go	86.66%	3 Missing and 3 partials ⚠️
pkg/skills/lockfile/validation.go	92.30%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5892      +/-   ##
==========================================
- Coverage   71.63%   71.61%   -0.02%     
==========================================
  Files         698      701       +3     
  Lines       71541    71746     +205     
==========================================
+ Hits        51245    51383     +138     
- Misses      16598    16668      +70     
+ Partials     3698     3695       -3     

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[samuv]

Tracked in #5899 (RFC THV-0080 stack).