💡 Change Scope

• Small Change: Fast feedback, localized UI/logic, low-risk.
• Large Change: New feature, cross-boundary logic, runtime-risk (persistence, IPC, lifecycle, recovery).

📝 What Does This PR Do?

Adds native system notifications for agent completion/standby events while preserving the existing in-app banner behavior.

• Adds a validated system:show-notification IPC path and preload/browser API.
• Reuses the existing agent standby notification watcher so completion events remain deduped by session.
• Formats native notification text with workspace, task, and space context.
• Suppresses real OS notifications in test runs to avoid CI/E2E notification popups.
• Updates notification settings copy from banner-only wording to completion alert wording.

Cross-platform behavior

This is intended to work on all three supported desktop platforms: macOS, Windows, and Linux.

• Desktop builds use Electron Main process Notification, which routes through the OS notification system on macOS, Windows, and Linux.
• The API returns { shown: false } when Electron reports notifications are unsupported, so unsupported Linux desktop/session setups fail quietly instead of breaking agent completion handling.
• Linux support still depends on the user's desktop notification service/session configuration, as with other Electron native notifications.
• Browser/Web UI fallback uses the Web Notifications API.

I validated the implementation locally on macOS and kept automated tests from emitting real system notifications. Windows/Linux runtime behavior should be checked in platform CI or by maintainers with those desktop environments because this PR does not add platform-specific Windows/Linux E2E notification assertions.

🏗️ Large Change Spec (Required if "Large Change" is checked)

Not applicable. This is a small, localized notification-path change. It does not modify durable state, recovery semantics, or persistence schema.

1. Context & Business Logic

Agent completion is already detected when a session transitions from working to standby. This PR keeps that detection and adds an OS-native notification through Main process IPC.

2. State Ownership & Invariants

• Renderer owns the existing notification list and session dedupe.
• Main owns OS notification delivery.
• Invariant: a session should only enqueue one completion alert for the same standby transition.
• Invariant: tests must not emit real system notifications.

3. Verification Plan & Regression Layer

• Unit test covers native notification text formatting.
• Typecheck covers the new IPC/preload DTO wiring.
• Existing E2E coverage confirms the standby notification path still works.

✅ Delivery & Compliance Checklist

• My code passes the ultimate gatekeeper: pnpm pre-commit is completely green.
• I have signed the CLA if required (see CLA.md).
• I have included new tests to lock down the behavior (or explicitly stated why it's untestable).
• I have strictly adhered to the DEVELOPMENT.md architectural boundaries.
• I have attached a screenshot or screen recording (if this touches the UI).
• I have updated the documentation accordingly (if adding a feature or changing a contract).

📸 Screenshots / Visual Evidence

Not attached. The native OS notification is suppressed in automated test mode; the existing in-app banner path was verified by E2E.

Verification

• npx pnpm@9.6.0 test -- --run tests/unit/app/agentStandbySystemNotification.spec.ts tests/unit/contexts/agentConfig.spec.ts
• npx pnpm@9.6.0 check
• npx pnpm@9.6.0 lint
• npx pnpm@9.6.0 build
• npx pnpm@9.6.0 exec playwright test tests/e2e/workspace-canvas.agent-status-watcher.spec.ts --grep "updates an active agent"