Mobile preview channels: cleanup on PR close + in-app channel browser

Status summary: done. The two follow-ups deliberately left out of #2412: EAS channels/branches pile up forever once their PR closes, and discovering active channels requires finding the right PR body — the phone should just list them.

Decisions#

• Cleanup: new workflow on pull_request: [closed] over the same mobile paths → scripts/ci/cleanup-mobile-pr-preview.ts deletes the PR's EAS channel and branch (channelForBranch from scripts/ci/mobile-preview.ts keeps naming in lockstep) and the PR's QR assets from the gh-attach-assets release. Tolerant of absence — a PR whose preview never published must close clean.
• Channel list transport: an authenticated OS server route. The deployed worker already holds EXPO_TOKEN (deploy pushes the whole Doppler config as secrets), so it queries the EAS GraphQL API server-side; the repo is private so client-side/static alternatives (release assets, gists) don't work. Auth: a session-level itx method (Session.mobilePreviewChannels) — the phone already holds an authenticated session stub, so no new auth surface at all; the list is low-sensitivity (branch names) but not public.
• App screen: preview-channels.tsx listing active channels (name, latest update message, published-at), newest first, current override highlighted; tapping one routes to the existing preview-channel/[channel] confirm screen. Entry point next to Build info in the project drawer.

Checklist#

• scripts/ci/cleanup-mobile-pr-preview.ts + .depot/workflows/mobile-pr-preview-cleanup.yml
• OS route serving the channel list from EAS GraphQL built, then removed: Misha ruled out shipping EXPO_TOKEN to the deployment. Channel discovery stays on PR-body QRs; a CI-pushed snapshot could enable a tokenless in-app list later
• app: channels screen + drawer entry removed with the above
• tests: cleanup decision logic (pure, DI'd), channel-list response shaping; workflow schema test picks up the new yml
• README: per-PR channels section gains the lifecycle story

Notes#

• Depends on scripts/ci/mobile-preview.ts landing via PR #2416 (shared helpers); rebase or merge that branch if it hasn't merged first.

Implementation log#

• EAS GraphQL query verified live against api.expo.dev before baking in (channels → latest update group message/createdAt/runtimeVersion).
• EXPO_TOKEN reaches the worker via OPTIONAL_SECRETS + Env.EXPO_TOKEN (same pattern as R2 keys); method reports available:false without it.
• itx contract regenerated (apps/os + packages/iterate mirrors) via generate:itx-api.