fix(crosscheck): support credentialless private PR evidence

[ruby-dlee]

Intent

Repair Azure Crosscheck so exact private pull request evidence works credentiallessly across macOS and Linux, ship the repair, and leave Relvino PR 1476 uncleared until its exact-head retry returns an admitted verdict.

What Changed

• Package authenticated exact PR-head checkouts as digest-bound private Git bundles, allowing Azure Crosscheck evidence to run without GitHub credentials or repository networking.
• Validate bundled source ancestors locally on both the host and guest, and support external repositories without Firstmate’s Python lockfile.
• Resolve the credential-refusing askpass executable portably across macOS and Linux.

Risk Assessment

✅ Low: The credentialless private-bundle path is bounded by exact commit, ancestry, tree, bundle digest, and size checks; the portable askpass fix is behaviorally covered, and Relvino PR 1476 remains open.

Testing

• ⏭️ Test - skipped

Pipeline

Updates from git push no-mistakes

✅ **intent** - passed

✅ No issues found.

✅ **Rebase** - passed

✅ No issues found.

🔧 **Review** - 1 issue found → auto-fixed ✅

• ⚠️ tests/fm-azure-runner.test.sh:186 - This modified assertion only counts an implementation-source substring, so it violates the test-quality rule and will break on behavior-preserving refactors. Remove it; the private snapshot preparation tests already exercise the source-binding behavior through the executable interface.

🔧 Fix: Remove brittle assertion and repair ancestor fixture
✅ Re-checked - no issues remain.

⏭️ **Test** - skipped

Step was skipped.

✅ **Document** - passed

✅ No issues found.

⏭️ **Lint** - skipped

Step was skipped.

✅ **Push** - passed

✅ No issues found.