Project Egress Interception Uses Fetch Capabilities

The old intercept-route design was deleted once fetch became an ordinary shadowable itx capability. A live fetch cap intercepts all project egress with the same session-bound semantics and the same secret-withholding property (placeholders reach the interceptor unsubstituted). See apps/os/docs/itx-design.md §9.

OS no longer stores an external egress proxy URL on Projects. Tests and operator debugging pass a live fetch capability into the Project runtime instead. This keeps outbound interception scoped to one Project Durable Object, removes persistent proxy configuration from the product model, avoids external transport plumbing in e2e tests, and lets OS withhold Secret Material while still showing the original getSecret(...) incantation to the intercepting test.

Captun PR stack: https://github.com/iterate/captun/pull/1

Amendment (itx-v4 replacement): the decision carried over; the current mechanism is itx.egress.intercept(handler) (ProjectEgress.intercept in apps/os/src/types.ts) — a live, last-writer-wins replacement installed on the Project Durable Object with the same session-bound semantics and the same secret-withholding property.