feat(core): ntfy.sh notification dispatcher module

Adds a transport-agnostic NotificationDispatcher and a fire-and-forget
ntfy.sh transport (no SDK; just fetch). Configuration is persisted as a
single global JSON blob under the 'ntfy_config' settings key.

Event taxonomy (per-event toggles):
- turn-completed   — assistant turn finished cleanly
- turn-error       — final turn error (after all fallbacks)
- permission-required — new permission prompt was created
- agent-spawned    — top-level user-agent tab spawned via 'summon'

Design:
- Single internal notify(event) interface so a future transport (email,
  webhook) plugs in without changing call sites.
- attachToAgentManager + attachToPermissionManager subscribe to the
  existing event streams via narrow listener interfaces (no @dispatch/api
  dependency back into core).
- 5s in-memory dedupe window on dedupeKey suppresses permission re-emits.
- 10s per-request abort timeout so a hung ntfy server can't pin a worker.
- All sends are fire-and-forget: void Promise.resolve(...).catch(warn).

Tests (39 new):
- ntfy transport: URL/headers/body/auth/click, header sanitization,
  per-event-type defaults, error paths.
- config: defaults, normalization tolerance, round-trip, redaction.
- dispatcher: master switch, per-event toggle, dedupe, agent/permission
  hookups, top-level-only filtering for agent-spawned, dispose.

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