Deduplicate Hermes session scan across workspaces

[luckeyfaraday]

Summary

readHermesSessions(workspace) re-ran the entire workspace-independent Hermes corpus scan — the ~/.hermes/state.db query plus a full readFile + JSON.stringify of up to MAX_PROVIDER_ROWS (1000) session files — and only narrowed to the workspace with a final match. PR #154 ("Improve multi-workspace review handoffs", first shipped in v0.1.9) began refreshing agent sessions for every open workspace tab, so an N-workspace review fanned this single scan into N concurrent full re-reads of the same corpus, multiplying peak heap by the workspace count.

This is the Hermes analogue of the codex amplification fixed in #169, and the last unbounded per-workspace session-scan path flagged in the v0.1.9 heap-OOM investigation.

Change

• Add memoizeAsyncWithTtl (client/electron/ttl-cache.ts): a small async memoizer that shares one in-flight promise per TTL window and never caches rejections — the same semantics as the existing inline codex metadata cache.
• Split readHermesSessions into:
  • scanHermesSessions() — the workspace-independent scan, now run once per CACHE_TTL_MS (30s) and shared across all workspaces.
  • readHermesSessions(workspace) — filters the shared result with hermesSessionMatchesWorkspace and stamps the per-workspace workspace field.

Eleven open workspaces now perform one corpus scan per 30s window instead of eleven concurrent ones. The session-building logic is otherwise unchanged.

Behavior notes

• No-database fallback: the on-disk fallback scan previously triggered when the DB produced no rows for the current workspace; hoisted to the shared scan, it now triggers when the DB produces no rows at all. For the common case (every session present in state.db) the result is identical.
• Trade-off: the shared scan retains parsed metadata (including each session's ≤300 KB searchText, which the per-workspace match needs) for the 30s TTL. This is bounded by the existing MAX_PROVIDER_ROWS × 300 KB cap and is far below the multi-GB transient spike it replaces.

Verification

• npm run build:electron — clean
• npm run test:electron — 133/133 pass, including 3 new ttl-cache tests (in-flight sharing, TTL expiry, rejections not cached)

Context

Root cause: PR #154 (b4c801c) turned one active-workspace session scan into one-per-workspace. The codex/claude side is bounded in #169; this PR closes the Hermes side.

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