Mobile: per-PR preview channels with in-app switcher and PR-body QRs

Status summary: done, verified end-to-end (this PR body carries its own live QR section, produced by running the CI script locally against PR #2412). Kills last-write-wins on the shared preview channel: PRs publish to their own channel, the app can switch channels via deep link, and every mobile PR gets tappable QRs in its body.

Today every publisher (merge-to-main CI, PR-branch agents) writes to the one preview channel, so whatever published last is what's on Misha's phone — his #2410 preview got silently replaced by an unrelated main merge within minutes. Decided 2026-08-04: per-PR channels, in-app switcher, QRs on PR bodies.

Design#

• preview channel becomes main-only. PRs publish to a channel named after their branch (sanitized: non [a-z0-9._-] chars → -).
• Channel switching uses Updates.setUpdateRequestHeadersOverride({"expo-channel-name": ...}) — the headers-only override, which does not need disableAntiBrickingMeasures (override keys only have to exist in the embedded headers, and EAS builds embed expo-channel-name). Persisted natively in UserDefaults across restarts.
• Deep link iterate://preview-channel/<channel> opens a confirm screen (no auto-switch on link open) that applies the override, fetches, reloads.
• A managed PR-body section (markdownAnnotator, like loc-report) shows both QRs in <details> blocks for any PR touching apps/mobile/** — OTA deep-link QR and full-install QR — with the heuristic picking which is expanded: PR runtime matches the latest finished preview build → OTA open; otherwise install open (OTA still present: Misha may have installed this branch's native build earlier). QR PNGs upload to the existing gh-attach-assets release; images render inline from release URLs, wrapped in <a> so they're tappable when reading the PR on the phone.
• Native-change case: find a usable build for the PR's runtime, trigger one (--no-wait) if none.

Checklist#

• apps/mobile/src/lib/preview-channel.ts: set/clear channel override + AsyncStorage mirror (expo-updates has no getter)
• apps/mobile/src/app/preview-channel/[channel].tsx: deep-link confirm screen (switch → check → fetch → reload; friendly "nothing published/incompatible" outcome)
• Build info screen: show active channel override + "reset to default channel" action
• write-build-info.mjs: prefer GITHUB_HEAD_REF (PR head branch) over GITHUB_REF_NAME for the branch stamp
• scripts/ci/publish-mobile-pr-preview.ts: publish PR channel, decide js-only vs native, ensure build exists for native, generate + upload QRs, write PR-body section
• .depot/workflows/mobile-pr-preview.yml: pull_request on apps/mobile/**, per-PR concurrency (cancel-in-progress), contents:write + pull-requests:write
• unit tests for the decision + rendering logic (pure functions, DI'd exec — no mocks)
• README blurb in apps/mobile/README.md

Notes#

• Stacked on mobile-fingerprint-sourceskips (PR #2411) — without sourceSkips, script-only package.json edits would misclassify PRs as native.
• Follow-up (not here): clean up EAS channels/branches when PRs close; an in-app list of open PR channels (needs an authed EAS API proxy).

Implementation log#

• e2e run against PR #2412 caught --status casing: the eas-cli flag wants lowercase (finished) while its JSON output reports uppercase (FINISHED).
• The runtime-mismatch path exercised for real: this branch changes the fingerprint (sourceSkips config from #2411), so the run triggered native build 9623bb11 and expanded the install QR — exactly the heuristic working.
• The PR-triggered build shares the post-merge fingerprint, so the merge-to-main workflow will find it and skip its own build.