Add gitsign commit-signature verification

[samuv]

Note

Stack 2 of RFC THV-0080 (tracking issue #5899) — stacked on #6091 (verifier wrapper); merges after it, into it, or rebases onto main once #6091 lands. This completes the git half of the verification story that #6084 deliberately deferred.

Summary

Git-installed skills need the same install-time verification as OCI artifacts. This PR adds Verifier.VerifyGit, full cryptographic verification of gitsign commit signatures:

• CMS verification over the commit payload: the armored PKCS#7 signature is parsed and verified (message digest + signature over signed attributes) via github/smimesign/ietf-cms — the same library gitsign wraps. Importing gitsign's own pkg/git was evaluated and rejected: its package graph compiles cosign and the GCP/Azure/AWS KMS SDKs into the binary (~54 new module entries). ietf-cms was already in our dependency graph.
• Chain of trust to Fulcio: the signing certificate chain is verified against the Fulcio roots from toolhive-core's embedded trusted material — hermetic, no network.
• Identity extraction reuses core's normalization (GitHub Actions workflow paths, issuer OID) by summarizing the certificate through sigstore-go's SummarizeCertificate and core's IdentityFromResult.
• Expected-identity enforcement is an explicit comparison (ErrSignerMismatch): unlike the OCI path there is no Sigstore bundle to bind a policy into. Nil expected is trust-on-first-use, same as OCI.
• Payload plumbing: git.HeadCommit and gitresolver.ResolveResult gain the signed payload (encoded commit minus the signature header — the exact bytes the signature covers), so the install flow can hand the verifier signature + payload from one lookup.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Test improvements
• CI/CD or build system changes

Test plan

• Unit tests pass locally (task test on the stack tip)
• Linting passes (task lint-fix, 0 issues)
• New unit tests: full round trip with a synthetic Fulcio-style CA (real CMS signatures produced by the same ietf-cms package, real X.509 chains) — verify, tampered-payload rejection, untrusted-root rejection, garbage-signature rejection, identity extraction (SAN + Fulcio issuer OID), expected-identity match/mismatch matrix, unsigned/empty-payload guards; payload coverage in the git client and resolver tests (signed payload excludes the gpgsig header, present for unsigned commits too). Zero network.

Does this introduce a user-facing change?

No — nothing calls VerifyGit yet; enforcement arrives with the install-verification PR.

Special notes for reviewers

• Signing-time semantics: verification time is anchored inside the signing certificate's own validity window, matching gitsign's verifier (its fork of ietf-cms does the same per-signer). Fulcio certs live ~10 minutes, so "valid at time.Now" would reject every real signature. Proving the actual signing time is the transparency log's job: validating the Rekor proof embedded in CMS unsigned attributes is a tracked follow-up, because the reconstruction helpers live in gitsign's internal/ and reimplementing them pulls in the cosign dependency this PR deliberately avoids. Flagging explicitly since the approved plan wanted Rekor validation in-stack — this PR ships the signature+chain half; happy to discuss whether the follow-up should block PR11.
• The synthetic-CA test fixtures produce real signatures and real chains; only the trust anchor is test-local (verifyGitSignature takes injectable pools; the exported VerifyGit pins the embedded Fulcio roots).

Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 67.01031% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.52%. Comparing base (dafa3d2) to head (1cb4e00).

Files with missing lines	Patch %	Lines
pkg/skills/verifier/git.go	68.29%	19 Missing and 7 partials ⚠️
pkg/git/client.go	53.84%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@                    Coverage Diff                     @@
##           skills-sig/08-verifier    #6121      +/-   ##
==========================================================
+ Coverage                   72.41%   72.52%   +0.11%     
==========================================================
  Files                         737      739       +2     
  Lines                       75960    76113     +153     
==========================================================
+ Hits                        55005    55200     +195     
+ Misses                      17055    16984      -71     
- Partials                     3900     3929      +29     

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[JAORMX]

Panel-review comment on the git half of the verification stack (reviewed together with #6091 and #6129 against RFC THV-0080). One finding, inline: the missing Rekor inclusion-proof validation means the cert-validity anchor at CurrentTime: NotBefore+1min never expires, so the git path's replay window is unbounded — a historical Fulcio cert verifies forever. The PR body already flags this; the inline comment argues for landing it as visibly provisional (marked in the lock like unsigned exceptions) rather than blocking #6129 on vendoring gitsign's Rekor helpers, plus a test that pins current behavior with a TODO to flip when Rekor lands.

Everything else in this PR checked out clean: the smimesign/ietf-cms dependency choice over gitsign's pkg/git (cosign + cloud-KMS closure) is right, the cmsMu serialization of ietf-cms's global BER state is documented and correct, identity extraction reusing core's normalization via SummarizeCertificate avoids re-implementing the GitHub Actions path rules, and the synthetic-CA tests produce real signatures/chains with zero network.