feat(comms): P1 presence — minimal Synapse + fleet presence room + mosaic.presence heartbeat + liveness (#888)

Co-authored-by: jason.woltje <jason@diversecanvas.com>
Co-committed-by: jason.woltje <jason@diversecanvas.com>

packages/comms/README.md

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+# @mosaicstack/comms
+
+MACP presence SDK — the **P1 (presence)** slice of RFC-001 (§4.5 liveness,
+§4.2 event envelope). Minimal by design: set Matrix presence, run the
+`mosaic.presence` heartbeat, and compute **deterministic** fleet liveness.
+
+Out of P1 scope (later phases): enrollment/auto-detect, room taxonomy,
+per-agent token minting, signed-authorship, federation.
+
+## API
+
+- `classifyLiveness(ageMs, policy)` / `computeFleetLiveness(observations, now, policy)`
+  — pure, deterministic online/away/offline from heartbeat age. The
+  authoritative liveness source (RFC-001 §4.5): native Matrix presence is _not_
+  relied upon.
+- `HeartbeatEmitter` / `startHeartbeatLoop(...)` — build and drive the
+  `mosaic.presence` heartbeat (monotonic `seq`, `interval_ms`).
+- `MinimalMatrixClient` — tiny C-S client: `setPresence`, `sendHeartbeat`,
+  `readHeartbeats`, `joinRoom`. Supports Application-Service masquerade
+  (`actAsUserId`) for the P1 provisioner, or a per-agent `accessToken`.
+- `PresenceAgent` — high-level: join the fleet room, go present, heartbeat.
+  `pauseHeartbeat()` models a crash (no graceful signal).
+- `FleetLivenessReader` — reads the fleet room and computes the liveness board
+  (`read()` / `formatBoard()`), the surface a human or watchdog reads.
+
+## Liveness policy (RFC-001 §4.5)
+
+```
+online : age <= heartbeatIntervalMs * missTolerance
+away   : age <  darkThresholdMs
+offline: otherwise (or never-seen / non-finite age -> fail safe to offline)
+```
+
+Defaults: interval 30s, miss-tolerance 2, dark-threshold 10min
+(`DEFAULT_LIVENESS_POLICY`). All runtime-tunable per RFC-002 §5.3.
+
+## Tests
+
+`pnpm --filter @mosaicstack/comms test` — the liveness core is written
+RED-FIRST; an end-to-end proof against a real Synapse lives in
+`tools/matrix-presence-harness`.