feat(notify): add explicit notification service and durable outcomes

[777genius]

Adds the explicit notification service behind injected policy, origin, readiness and delivery ports. A request is durably admitted before delivery; replays preserve the original target and the actual terminal navigation/suppression outcome without resending. Configurable limits use the shared journal transaction.

Stacked on #158. This is the service checkpoint; CLI/MCP activation and installed Desktop qualification remain later steps.

Validation: exact-source independent follow-up review found no actionable issues; focused Linux unit/race/vet passed as a non-root user; Mac service/journal/origin suites passed. The combined real SDK -> service -> journal fixture also passes with mocked OS delivery. No installed notification or release claim.

The coherent patch is 2,187 changed lines, slightly above the review target; most additions are regression tests for the single admission/outcome invariant. Splitting terminal persistence from service validation would leave an inconsistent intermediate contract. Old writers must be fenced before activation; journal history is never reset.

[coderabbitai]

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