fix(notification): prevent replayed task notifications

[yg2224]

session: issue911-notification-duplicate

Fixes #911

Summary

• make task-native notifications exactly-once by durable notification id
• close native task objects after successful read, mark-all-read, and clear actions
• retain bounded tombstones and request-time acknowledgement watermarks so delayed renderer events cannot replay dismissed rows
• guard renderer notification refreshes against stale async responses and preserve host unread totals/session outcome state
• keep interactive prompt notifications on their independent lifecycle

Validation

• node --test apps/desktop/test/notification-contract.test.mjs apps/desktop/test/notification-ipc-dismissal.test.mjs — 14 passed
• pnpm --filter @pi-desktop/desktop typecheck — passed
• pnpm --filter @pi-desktop/desktop test — passed
• pnpm build:js — passed
• pnpm lint — passed
• pnpm docs:check — passed
• cargo fmt --check — passed
• cargo test -p host-core --locked — 643 passed
• cargo clippy -p host-core --all-targets — passed with 4 pre-existing warnings
• integrated-main candidate f58c6d8d7: pnpm test:e2e — 23 passed, 2 expected skips (no PI_DESKTOP_TEST_API_KEY)
• integrated-main candidate f58c6d8d7: pnpm test:e2e:boot — passed

Remaining environment limitation

E2E: NOT RUN
Suite: packaged Windows Action Center native-notification activation
Reason: the provided Windows Tailscale SSH endpoint timed out, and this Mac environment has no Windows GUI automation session.
Alternative validation: native IPC contract tests, desktop test suite, host tests, and integrated smoke/boot E2E passed.
Remaining risk: Windows-specific Action Center attribution and click activation still need a Windows runner.

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