The next repo-local sweep target was ROADMAP #64: downstream consumers
still had to infer artifact provenance from prose even though the repo
already emitted structured lane events. The fix extends `lane.finished`
metadata with structured artifact provenance so successful completions
can report roadmap ids, files, diff stat, verification state, and commit
sha without relying on narration alone.
Constraint: Preserve the existing commit-created event and lane-finished metadata paths while adding structured provenance to successful completions
Rejected: Introduce a separate artifact event type first | unnecessary for this focused closeout because `lane.finished` already carries structured data and existing consumers can read it there
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: If artifact provenance extraction rules change later, update `extract_artifact_provenance`, its regression payload, and the ROADMAP closeout together
Tested: cargo fmt --all --check; cargo clippy --workspace --all-targets -- -D warnings; cargo test --workspace; architect review APPROVE
Not-tested: Downstream consumers that ignore `lane.finished.data.artifactProvenance` and still parse only prose output
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