Find and fix the root cause of the Ready pipeline bottleneck. #11836

Root cause is right: Ready age was measured from issue opening, so a freshly unblocked or just-finished issue read as weeks old. since is now the latest of opening, last blocker close, and last agent run end. The correlated subqueries use run_issue, so no new cost. The test fails without the change and is read-only for the UI. Single-purpose, no new dependency, no web/TUI surface, so no doc or both-doors change is needed.

Two things I would do next:

1. In test/flow-ready-since.test.ts the final assertion ended > closedAt ? ended : closedAt re-implements the logic. Assert the literal ended. The comment says the run ended two hours ago but the value is 30 minutes. Fix the comment.
2. The Ready tile shows only a number. Say in docs/features what Ready age means now (since unblocked or last run), so nobody reads it as time since opening.