#2794 gatorwalk-factory: a built-in tracker core that the Lab and Linear adapters build on

Part of the tracker-seam decision (https://claude.ai/artifact/TDidgyVjxKXU7JsNH1baNy). Needs #2793.

Every lifecycle should have a tracker, even with no external one. Today a work item can only reach a ticket through the Linear or Lab adapter, and a project with no external tracker has no issues at all.

Change

• tracker/core keeps issue records, issue-<id>, with origin: builtin | snapshot. snapshot means an external tracker owns the facts (identity, title, type, relations, assignment) and the record is its last read, with fetchedAt. builtin means the core owns them.
• A backend interface in core. The current TrackerAdapter (_lib/tracker.ts:92-103) gains create(draft), and capabilities become an explicit object; history is kept as a capability.
• A new model type, @swamp/gatorwalk-factory/tracker: the built-in backend, implementing the whole contract against swamp data.
• Built-in ids are <PREFIX>-<slug>-<rnd>, using the work-item key rules (DESIGN.md "The model types", :850). There is no counter, so ids stay unique across several users. The prefix is a tracker argument until the lifecycle-level configuration lands.
• swamp-club and linear are rebuilt on the core. With an integration, create goes to the external tracker first and its id is used.
• A missing capability is refused as TrackerError invalid naming the capability, never degraded silently.
• The conformance suite (_lib/tracker_conformance.ts) runs against all three backends.

Decide during planning

• Whether a built-in issue's first work item reuses the issue's slug as its key, rather than claim prefixing the display id onto a new slug (which would give cue-foo-r2ne-foo-x7ab).

Done when

• create, fetch_issue, comment, set_status, claim and publish work against the built-in tracker with no external service, and pass conformance.
• The Lab and Linear adapters pass conformance unchanged against their fakes.
• DESIGN.md "Trackers" describes the core, the two record origins, and built-in ids.