Ignore superseded Decodex run attempts

[yvette-carlisle]

Summary

• classify stale active or exited run attempts as superseded when a newer attempt exists for the same issue
• reconcile superseded runs without tracker/Linear writeback or stale worktree cleanup
• clear stale retry scheduling for superseded daemon children

Validation

• cargo +nightly fmt --all -- --check
• cargo test -p decodex exited_child_reconciliation_ignores_superseded_failed_run
• cargo test -p decodex active_run_reconciliation_supersedes_stale_lease_for_newer_attempt
• cargo test -p decodex reconciliation
• cargo test -p decodex retry
• cargo make check