Goal

Add a read-only global-sync accessor for cache-only call sites that need to inspect an existing child store without creating, bootstrapping, or pinning a directory store.

The immediate motivating case is PR #471: sidebar timestamp calculation can safely read already-loaded message cache, but it should not need to expand child-store lifecycle just to discover that a directory cache is absent.

Scope

In scope:

• Investigate current globalSync.child(...), children.child(...), and children.peek(...) semantics.
• Add a clearly named accessor for "existing child store only" if the investigation confirms cache-only callers need it.
• The accessor should return undefined or an equivalent safe empty result when the child store does not already exist.
• It must not call ensureChild(...) for missing directories.
• It must not bootstrap network/project loading.
• It must not pin the store to the current owner.
• Update sidebar timestamp cache reads to use the accessor if it exists and keeps fix(app): use last user message time in sidebar #471's fallback semantics.
• Add focused tests proving missing directories are not created and existing message caches can still be read.

Out of scope:

• Do not change session list ordering or pagination. That is tracked in [Feature] Sort initial sidebar sessions by last user message time #473.
• Do not change persistence schema, backend APIs, desktop shell behavior, or message cache storage.
• Do not replace all globalSync.child call sites mechanically. Audit each cache-only caller separately.
• Do not weaken existing child() behavior for normal callers that intentionally create/bootstrap/pin stores.

Relevant files or context

Relevant files:

• packages/app/src/context/global-sync/child-store.ts
• packages/app/src/context/global-sync/types.ts
• packages/app/src/context/global-sync.tsx
• packages/app/src/context/sync.tsx
• packages/app/src/pages/layout.tsx
• packages/app/src/pages/layout/pawwork-session-source.ts

Related:

• PR fix(app): use last user message time in sidebar #471 uses globalSync.child(session.directory, { bootstrap: false, pin: false }) for sidebar timestamp reads. That avoids bootstrap and pinning, but still creates/touches an unpinned child store because child() calls ensureChild(...).
• [Feature] Sort initial sidebar sessions by last user message time #473 tracks the separate product/API problem of selecting the initial sidebar window by latest user message time.
• [Task] Split layout.tsx into agent-readable controllers #474 tracks broader layout.tsx controller extraction and should not be blocked on this low-priority cache accessor unless the same area is being touched.

Verification

• Add or update unit tests for child-store manager behavior showing the new accessor does not create a missing child store.
• Add or update a layout/sidebar timestamp test showing missing cache still falls back to session.time.created.
• Run bun --cwd packages/app test --preload ./happydom.ts with the targeted test files.
• Run bun --cwd packages/app typecheck.
• Run git diff --check.

Execution mode

Agent should investigate and propose a plan first