feat(notify): add durable explicit notification journal

[777genius]

Adds the durable admission journal for agent-triggered notifications. Requests are scoped by source, session and key kind; an admission is persisted before delivery, and ambiguous outcomes remain replayable without granting another effect.

The package provides bounded snapshots, permanent kernel locking, conservative seven-day retention, atomic rate limits, attempt-token finalization and read-only lookup. Service/CLI wiring is the next integration step.

Stacked on #153 (feat/agent-notify-native-protocol). This is the journal portion of phase 4, prepared independently under the plan's parallel-work allowance. The 2,075 added lines keep the persistence invariant and its process/crash tests together; 1,007 lines are tests.

Validation:

• Independent source review: no actionable findings; source hashes verified before and after review.
• Linux focused tests, vet and race tests passed; Windows unsupported-platform path compiles.
• macOS package tests passed, including real child-process crash/concurrency fixtures; the Darwin kernel clock qualification passed separately.
• These checks cover journal behavior. Native delivery, installed Desktop E2E and release qualification remain separate gates.

[coderabbitai]

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