Classify quiet agent states before they look stale

Persist derived machine states for agent manifests so downstream monitors can distinguish working, blocked, degraded, and finished-cleanable lanes without inferring everything from prose. This also records commit provenance in terminal-state manifests and marks the new session-state classification roadmap item as done.

Constraint: Keep the change scoped to manifest persistence and tests without introducing a new monitoring service layer
Rejected: Leave state classification as downstream text scraping only | repeated dogfood runs showed quiet/finished lanes being misreported as stale
Confidence: medium
Scope-risk: narrow
Directive: Reuse derived_state + commit provenance from manifests before adding any new stale-session heuristics elsewhere
Tested: python .github/scripts/check_doc_source_of_truth.py
Tested: cd rust && cargo fmt --all --check
Tested: cd rust && cargo test -q -p tools
Tested: cd rust && cargo clippy -p tools --all-targets --no-deps -- -D warnings
Not-tested: full cargo clippy --workspace --all-targets -- -D warnings still fails on unrelated pre-existing runtime lint debt

rust/crates/tools/src/lane_completion.rs

@@ -108,6 +108,7 @@ mod tests {
             started_at: Some("2024-01-01T00:00:00Z".to_string()),
             completed_at: Some("2024-01-01T00:00:00Z".to_string()),
             lane_events: vec![],
+            derived_state: "working".to_string(),
             current_blocker: None,
             error: None,
         }