One flow, named: a pure src/flow module says which stage each issue is in (issue → pull → check → review → land → live) and how long it has been there #1474

From #334 (1): flows are first-class. This is the model. The pages come after it.

Done looks like.

• [ ] src/flow/stage.ts is a pure function from an issue's existing records (its pulls, checks, reviews, merge, release/deploy) to {stage, since, stalled, unblock}. stage is one of issue, pull, check, review, land, live. stalled is true past a per-stage threshold. unblock is the single action that moves it on (e.g. "re-run the check", "review pull #N", "assign a builder").
• [ ] GET /api/v1/repos/:owner/:name/flow returns that for every open issue, plus anything that went live in the last 24 h, in under 50 ms on this repository's size.
• [ ] test/flow-stage.test.ts, named in GATE, covers each stage, each stall, and a pull that was closed and re-opened.
• [ ] Before the pull, bun test test/flow-stage.test.ts test/check-list.test.ts and bunx tsc --noEmit pass in the sandbox.

Files. src/flow/stage.ts (new), src/api routes, scripts/gate.ts, test/flow-stage.test.ts (new). Fits: scout.

Parked by the crew: the check failed ("acting on many asks at once > the card's one request answers its script with the region and the count; one undo link stays for a day") and the decider saw no point in another try (pull #1479 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.