fix: derive runtime source commit from observed product head

[ozand]

Summary

• Fixes runtime source provenance for archive/deployed runtimes that cannot read local git metadata.
• Adds fallback from workspace/state/self_evolution/current_state.json.observed_product_head.commit into _runtime_source_fingerprint().
• Preserves env and git provenance precedence; only uses observed product head when env/git commit is unavailable.
• Adds regression for the eeepc deployed-runtime shape that caused source_commit_missing.

Fixes #456

Test Plan

• python3 -m pytest tests/test_runtime_coordinator.py::test_runtime_source_fingerprint_falls_back_to_observed_product_head_when_git_is_unavailable tests/test_runtime_coordinator.py::test_runtime_source_fingerprint_prefers_explicit_release_env_for_archived_runtime -q → 2 passed
• python3 -m pytest tests/test_runtime_coordinator.py tests/test_promotion_workflow.py -q → 67 passed
• python3 -m pytest tests -q → 696 passed, 5 skipped

Live follow-up required after merge

• Deploy/restart local cycle/dashboard collection.
• Run one cycle with current_state observed product head available.
• Verify /api/mission-control.headline no longer reports Blocked: source_commit_missing for the active candidate, or report the next blocker if promotion advances.