#2805 gatorwalk-factory: saved scenarios next to a lifecycle, run by validate

Part of the gatorwalk-factory studio (gatorwalk-factory/ in swamp-extensions). Read gatorwalk-factory/README.md and DESIGN.md first. Design proposal (decisions recorded 2026-09-30): https://claude.ai/artifact/BNCKp7w3G3Q8VdmnCDDWHP. Prototype, with its source under source/ (read it with the Artifact tool): https://claude.ai/artifact/1wAxrpUqZ47FAjE4fU9idD. Both links are private to the maintainer.

Goal

A lifecycle can be tested before anyone runs work on it. Saved scenarios sit next to the lifecycle, and validate runs them in-process on the real engine, so CI and any author catch a lifecycle change that breaks a known path. For now, in-process on committingStore(memoryStore()) is good enough; revisit with data.

Format

lifecycles/<name>.scenarios/<scenario>.yaml, where <name> is the lifecycle file's basename. One engine call per step:

scenario: plan-churn
lifecycle: swamp-extensions
description: The fifth plan revision hits the cycle limit; a person grants one more pass.
externalRefs: { swamp-club: "2801" }
steps:
  - record: { evidence: classification }
    payload: { type: feature, confidence: high, reasoning: A new list method }
  - move: feature
  - move: revise
    manual: true
    expect: { refused: "cycle override for 'plan'" }
  - wait: 3600
    note: The run sits at the cycle limit until someone looks
  - override: { stage: plan, note: one more pass }
  - approve: abandon-confirmation
  - move: abandon
  - expect: { stage: abandoned }

• Verbs: record (artifact or evidence, with payload), approve, decline, move (manual: true is a person's go), override (a cycle override), wait (seconds on the simulated clock), and a bare expect: { stage }.
• expect: { refused: "<text>" }: the step must be refused, and its reason must contain the text (an empty string means any refusal). This pins gate messages.
• Clock: one second per engine reading plus each wait. Actors: records and automatic moves are an agent; approvals, overrides and manual moves are a person.

Scope

• _lib/scenario.ts: a zod schema for the file, and the runner. Start from the prototype's source/src/sim.ts, which already passes both swamp-extensions scenarios on the real engine. The runner must stay free of Deno APIs, because the studio runs it in the browser. It returns one frame per step: the committed run, readiness from evaluateTransitions, computeMetrics, and the outcome.
• validate runs every scenario of the holder's lifecycle and reports each unexpected step with its file and step index. Any unexpected step fails validate.
• Convert the pure walks in lifecycles_test.ts into scenario files that the test runs. The prototype's source/scenarios/swamp-extensions.bug-to-done.yaml is the "a bug walks triage to done" test, transcribed. Keep in code only the assertions that inspect CEL results.
• Document the format in the README and DESIGN.md ("Saved scenarios"), and in the skill: the agent writes scenario files (decided 2026-09-30), so the skill says where they go, the verbs, and to run validate after writing one.

Acceptance

• validate on swamp-extensions runs its scenarios and passes; breaking a gate message makes it fail and name the step.
• Unit tests cover each verb, expect both ways, and a malformed file.

Needs: #2803. Blocks: Simulate mode.