#2804 gatorwalk-factory: description fields on gates and work; move lifecycle comments into fields

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

Lifecycles are edited by agents and the studio, which rewrite the YAML, so comments do not survive. Everything worth keeping moves into description fields, which do survive and show in the design page and the studio. File-level comments that only restate the file are dropped.

Scope

• Add an optional description: string to every gate variant (beside type and config) and to work. It already exists on the lifecycle, stages, transitions, artifacts and evidence.
• Descriptions are author-facing. No engine path sends one to an agent today (dispatch.ts never reads them); add a test pinning that.
• designView carries the new fields (GateView.description, WorkView.description), and the design page shows them.
• Move the comments in lifecycles/swamp-extensions.yaml (47 comment lines) and lifecycles/build-swamp-extension.yaml (25):

• Also cover the testdata/lifecycles/*.yaml comments, and update lifecycles/swamp-extensions.md where it points at comments.

Acceptance

• No # comment lines remain in lifecycles/*.yaml, and the files still pass lifecycles_test.ts.
• The design page renders gate and work descriptions.

Needs: nothing. Blocks: Design mode.