fix: read release metadata for promotion source commits

[ozand]

Closes #456.

Summary

• Teach runtime source fingerprinting to read SOURCE_COMMIT/SOURCE_BRANCH/SOURCE_TREE release metadata from git-archive deployments before falling through to unrelated cwd git repos.
• This gives eeepc /opt self-evolving emitter an auditable source_commit even though pinned release trees do not include .git.
• Add regression coverage for archived-runtime release metadata precedence.

Test plan

• RED before fix: tests/test_runtime_coordinator.py::test_runtime_source_fingerprint_reads_release_metadata_before_git_for_archived_runtime failed; runtime returned local repo git HEAD instead of release metadata.
• python3 -m pytest tests/test_runtime_coordinator.py -k "runtime_source_fingerprint or promotion" -q -> 3 passed.
• python3 -m pytest tests/test_promotion_workflow.py -q -> 10 passed.
• python3 -m pytest tests -q -> 697 passed, 5 skipped.
• PYTHONPATH=ops/dashboard/src:ops/dashboard python3 -m pytest ops/dashboard/tests -q -> 165 passed.

Live follow-up after merge

• Deploy archived runtime to eeepc /opt with SOURCE_COMMIT metadata files.
• Trigger eeepc-self-evolving-agent-health.service.
• Verify /api/promotions latest promotion detail.provenance.source_commit and governance_packet.promotion_provenance.source_commit are non-null.