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

Implements RFC-001 P1 (the first shippable slice): deterministic Matrix
presence/liveness on a single-instance dev Synapse.

- infra/matrix/: DEV Synapse (RFC-002 Mode B, federation OFF, self-signed TLS,
  enable_registration:false, appservice registration wired). Rendered from
  parameterized templates — zero hardcoded topology. .data is gitignored.
- packages/comms/: minimal MACP presence SDK — set presence, run the
  mosaic.presence heartbeat (seq + interval per RFC-001 §4.5), and a
  deterministic liveness reader (online/away/offline from heartbeat age, NOT
  native-presence-timeout). Liveness core written RED-FIRST. 19 vitest tests.
- tools/matrix-presence-harness/: minimal provisioner (registers >=3 agent
  MXIDs, creates the fleet presence room, joins them — reuses the existing
  @mosaicstack/appservice intent lib) + an E2E validation harness proving
  A2/A3/A4 against a real Synapse.
- eslint.config.mjs: register packages/comms/vitest.config.ts with the
  type-aware project service (same as other packages' vitest configs).

DEV-compose validated only; production deploy is a separate coordinated step
(deploy-holds respected).

Part of the comms-evolution program (RFC-001 P1)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0158NZqN2n2ymKFeJAZ4GUCb

infra/matrix/dev-down.sh

@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+# dev-down.sh — tear down the DEV Synapse (matrix-p1-dev). DEV-ONLY.
+#   ./dev-down.sh            # stop + remove containers, keep ./.data
+#   ./dev-down.sh --purge    # also delete ./.data (fresh next bring-up)
+set -euo pipefail
+HERE="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+docker compose -f "${HERE}/docker-compose.dev.yml" --profile element down -v --remove-orphans || true
+if [[ "${1:-}" == "--purge" ]]; then
+  rm -rf "${HERE}/.data"
+  echo "[dev-down] purged ${HERE}/.data"
+fi
+echo "[dev-down] matrix-p1-dev stopped"