althing-chamber: add valkey for Phase 3.1 cross-process streaming

Phase 3.1 closes the cross-process gap the Phase 3 smoke surfaced —
streaming events (msg_start/thinking/delta/complete/curated) flow
from agent-runner → chamber via valkey pub/sub rather than the
SQLite bridge (too high-volume + ephemeral for the DB).

New service: `althing-valkey` (stock `valkey/valkey:8-alpine`).
Internal-only — no exposed port, no volume. chamber + agent-runner
reach via docker DNS at `valkey:6379` on the compose default
network. healthcheck via `valkey-cli ping` (5s interval). chamber
+ agent-runner gain `depends_on: valkey: service_healthy` so the
bridge is up before either side starts publishing or subscribing.

Forseti unchanged — never publishes Phase 3 events.

Operational properties (per forseti's deployment notes):
  - Mixed-state safe at every step. Missing valkey.url config key
    → chamber + runner stay on v3.0 / Phase 2 equivalent paths.
  - Backward path is single config-key delete + restart.
  - streaming_enabled: true (set on agent-runner 2026-05-17) is
    unaffected by this change.

README's services table + playbook header + verify section all
extended to reflect the four-service shape. Forseti's contract
at vh/althing:docs/contracts/phase3_1_valkey_bridge.contract.md
carries the wire-protocol spec.

stacks/althing-chamber/compose.yaml

@@ -1,17 +1,25 @@
-# althing-chamber + forseti + agent-runner — web UI + moderator daemon +
-# Worldtree-driver agent-runner for the althing inter-agent message bus.
+# althing-chamber stack — chamber + forseti + agent-runner + valkey.
 #
-# Three services share the althing SQLite store via a single bind-mount:
+# Four services on the compose default network:
 #   althing-chamber       — FastAPI/HTMX app (port 7881 host → 8000 container)
 #   althing-forseti       — moderator daemon (no port; cross-process glue via the DB)
 #   althing-agent-runner  — Phase 2 daemon: claims worldtree-driver grants and
 #                           dispatches to Worldtree's conversation API (no port)
+#   valkey                — Phase 3.1 sibling service: Valkey 8 alpine, redis-protocol
+#                           pub/sub bridge for cross-process streaming events from
+#                           agent-runner → chamber SSE subscribers. Reached via
+#                           docker DNS at `valkey:6379` on the default network.
+#                           No exposed port; internal-only.
 #
-# eventbus.bridge_from_db is load-bearing — forseti's DB commits reach
-# chamber's SSE subscribers via the bridge, not via shared memory or IPC.
-# Both processes read+write the same `~/.althing/althing.db` SQLite file
-# at /app/data/althing.db inside the container, backed by the host's
-# /opt/docker/conf/althing-chamber/data bind-mount.
+# Three storage / IPC channels:
+#   - SQLite bind-mount at /app/data backs the althing.db state shared by
+#     chamber + forseti + agent-runner (eventbus.bridge_from_db drains DB
+#     commits into chamber's SSE).
+#   - Valkey pub/sub on docker-default network carries the Phase 3 streaming
+#     events (msg_start/thinking/delta/complete/curated) that don't go
+#     through SQLite — too high-volume + ephemeral for the DB.
+#   - chamber's healthcheck-blocked startup gate on valkey ensures the
+#     subscriber side is up before chamber begins handling SSE traffic.
 #
 # Image is built on the host from the vh/althing git repo by the deploy
 # playbook (`playbooks/deploy-althing-chamber.yaml`), which clones into
@@ -26,10 +34,26 @@
 # All tunables live in .env — edit that, not this file.
 
 services:
+  valkey:
+    image: valkey/valkey:8-alpine
+    container_name: althing-valkey
+    restart: unless-stopped
+    # No exposed port — chamber + agent-runner reach via docker DNS
+    # at `valkey:6379` on the compose default network. No volume —
+    # the pub/sub channel doesn't persist anything between restarts.
+    healthcheck:
+      test: ["CMD", "valkey-cli", "ping"]
+      interval: 5s
+      timeout: 3s
+      retries: 5
+
   althing-chamber:
     image: ${ALTHING_IMAGE}
     container_name: althing-chamber
     restart: unless-stopped
+    depends_on:
+      valkey:
+        condition: service_healthy
     ports:
       - "${ALTHING_BIND:-0.0.0.0}:${ALTHING_PORT}:8000"
     environment:
@@ -81,6 +105,9 @@ services:
     image: ${ALTHING_IMAGE}
     container_name: althing-agent-runner
     restart: unless-stopped
+    depends_on:
+      valkey:
+        condition: service_healthy
     environment:
       - ALTHING_ROOT=/app/data
       - ALTHING_DB=/app/data/althing.db