Build info: show the commit message

Status summary: done. Three-file change (stamp script, placeholder, screen row), published to the preview channel as the first live test of the agent-PR → phone flow. Surfaced two publish-flow gotchas, noted below; one has a follow-up task.

The Build info screen shows branch/commit/author/time but not what the commit was. A hash means nothing at a glance; the subject line is the human-readable identity of the bundle. Add it.

stamp message (commit subject) in apps/mobile/scripts/write-build-info.mjs via git log -1 --format=%s, sliced to 200 chars; "" on EAS's .git-less archive like the other fields
add message to the checked-in placeholder apps/mobile/src/build-info.json
render a Message row in the Bundle section of apps/mobile/src/app/build-info.tsx between Commit and Built by
publish the branch to the preview channel so the installed app can pull it via Check for update published update 019fcd58-d5e5 (runtime 37fb004c, matching build 50ae8bb1) from commit 4adf339

Implementation notes#

Metro cache is poisoned across worktrees. The first eas update export failed because a use dom entry in the shared cache pointed at the root worktree's absolute path. --clear-cache fixes it; any worktree-hopping publish needs it.
eas update requires --message in non-interactive shells. The local update:preview script didn't pass one, so it only works from a TTY. Tried fixing with --message "$(git log -1 --format=%s)" in the script and hit the bigger gotcha:
The runtime fingerprint hashes package.json scripts. Editing update:preview changed the fingerprint from 37fb004c to f195c3d2, which would strand every installed binary despite zero native changes. Reverted the script edit from this PR to keep the flow test honest (update 019fcd57 on the orphaned runtime is harmless). Follow-up: add a fingerprint config with sourceSkips for package.json scripts, then land the --message fix — spawned as a separate task.