fix(build): auto-invalidate stale .materialized cache when overlay changes

[jeonghun-jj-lee]

Problem

build_app_bundle.mjs reuses .materialized when package.json exists, regardless of whether the overlay has changed since the last materialization. After an overlay sync (e.g. PR #1011 syncing the startsWith fix from opencode PR #337), a local rebuild silently ships the old bundle because the cached tree still contains the pre-fix source.

This is what caused the TypeError: Cannot read properties of undefined (reading 'startsWith') to persist in the installed extension even though the source code fix was merged and the overlay was synced.

Fix

Read manifest.json's fork_sha + promoted_at as the overlay version identity. Compare it against a .overlay-stamp file inside .materialized:

• Stamp differs or missing → wipe the cached tree and re-materialize
• Stamp matches → reuse the cache (existing fast path)
• Backfill — existing trees without a stamp get one written on first build (no unnecessary wipe)

The --direct-worktree and --verified-main modes are excluded from this check (they manage their own trees).

Testing

Verified all three paths locally:

Scenario	Result
No stamp (first build / old tree)	Detected stale, re-materialized, stamp written
Stamp matches manifest	Cache reused, no re-materialize
Stamp differs (simulated with fake stamp)	Detected stale, wiped, re-materialized

[coderabbitai]

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