feat(notify): support configurable journal admission rates

[777genius]

Explicit notifications need configurable limits shared across processes. Add a validated per-admission policy with the existing 6/session/minute, 30/runtime/minute and burst 3 defaults. Changes preserve accrued counters, stable request identity and replay; storage validation supports bounded expanded histories without quadratic rate scans.

Stacked on #156. This adds the journal contract; configuration activation and the compatible admission-writer floor are subsequent integration work. Old writers cannot safely enforce custom rates, despite unchanged disk JSON shape.

Validation: independent source review found no actionable issues; focused Linux package/race/vet passed with actual unprivileged process fixtures; macOS journal tests passed, including configured limits and contention. No native notifications or installed-feature completion is claimed.

[coderabbitai]

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