Yak ↔ external tracker — one-way projection

Yaks are a private, fine-grained, fast layer; external trackers (Jira, Linear, GitHub Issues) are a shared, coarse, slow layer. The relationship is a one-way projection, not synchronization: many yaks roll up to few external issues, the pointer runs yak → external only (each yak's source: URL), and the external tracker never knows yaks exist. There is no shared state, no field merge, nothing to reconcile.

See docs/design/projection.md for the full rationale. This skill covers the three things that need judgment; the rollup itself is just a CLI command.

Hard rules