feat(gateway): add sender presence tracking

docs/plans/2026-02-16-presence-tracking-checklist.md

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+# Presence Tracking Checklist
+
+Date: 2026-02-16
+Status: completed
+
+## Scope
+
+- Add runtime presence tracking for observed senders.
+- Expose presence via gateway API for dashboard/ops visibility.
+
+## Completed
+
+- Added presence tracking to `ChannelRegistry`:
+  - records inbound sender activity (`channel + senderId`)
+  - tracks `firstSeenAt`, `lastSeenAt`, `messageCount`
+  - infers `online|offline` from inactivity window
+  - supports filtering (`channel`, `status`, `limit`)
+- Added `system.presence` gateway method via system handlers.
+- Wired gateway server to source presence data from `ChannelRegistry`.
+- Added tests for registry presence behavior and handler output.
+- Updated docs for new API surface.
+
+## Verification
+
+- `pnpm test:run src/channels/registry.test.ts`
+- `pnpm test:run src/gateway/handlers/handlers.test.ts`
+- `pnpm typecheck`