#2800 gatorwalk-factory: a breakdown stage files child issues and their dependencies

Part of the tracker-seam decision (https://claude.ai/artifact/TDidgyVjxKXU7JsNH1baNy). Needs #2795 and #2796, and #2767 (for examples/).

Seth's call: breaking an epic down is an optional stage some lifecycles include, as they include a plan stage.

Change

• A stage's tracker block gains breakdown: { product: <name>, on: { approve: <gate> } }.
• The product's schema is { children: [{ ref, title, body, type?, blockedBy?: [ref] }] }, validated by the engine like any product and approved by a gate.
• On approval, publish (or an explicit file_breakdown method):
  1. creates each child through the tracker, in dependency order;
  2. records parent_of from the work item's issue to each child, and blocked_by for each edge;
  3. writes breakdown-<key> mapping each ref to its issue id.
• Every write is ledgered with the child's ref in the key, so a failure part-way resumes and a re-run files nothing twice.
• Children are never started automatically. Each is claimed like any other issue (claim warns while blocked).
• An example lifecycle with a breakdown stage in examples/, validated by the examples test.

Done when

• Against the built-in tracker and the Lab fake: approving a breakdown files children and edges once; a failure after the second child and a re-run files the rest; unknown refs in blockedBy and cycles fail product validation.
• The design page shows the breakdown stage.