sandbox: kitchen-sink images, feature-aware selection (Phase 2)

Phase 1 made the provisioner able to activate allowlisted extensions
post-ready but kept the bare upstream images. Phase 2 ships the images that
actually carry the extension/module files, and selects them only when a
venture requests features — bare sandboxes stay on the light upstream image
(no heavier pull, honoring the 'existing opters stay bare' decision).

- _PostgresEngine / _RedisEngine gain kitchen_sink_image + image_for(features):
  bare (no features) -> the light image; features requested -> the kitchen-sink
  image. The provisioner runs engine.image_for(features), not engine.image, so
  the bare path is byte-for-byte unchanged. Mongo inherits the base image_for
  (returns its image regardless — no activatable features).
- docker/sandbox-pg.Dockerfile: pgvector/pgvector:pg16 (ships vector) + postgis
  apt install; contrib (pg_trgm/citext/uuid-ossp) inherited from the official
  postgres base. Built at deploy via the sandbox-pg-image compose one-shot
  (mirrors the agent-image builders); the provisioner's _ensure_image finds the
  local tag and never pulls. Published by release.yml; pulled in registry
  compose. The verify step fails loudly if an extension's files are missing.
- _RedisEngine kitchen-sink image: redis/redis-stack-server:latest (headless;
  ships search/json/bloom as loadable-but-unloaded modules — no custom build).
- Extended the sandbox image-tag ghost-tag guard (the mongo:8-alpine regression
  test) to also cover kitchen_sink_image: skips locally-built roboco-* images,
  uses the namespaced Docker Hub endpoint for redis/redis-stack-server.

Image-specific package names / module .so paths are verified at the CEO's NAS
deploy (the spec's NAS smoke); the unit tests with the fake runner remain the
CI bar, and the verify step is the fail-loud safety net for a wrong build.

docker-compose.yml

@@ -383,6 +383,20 @@ services:
     depends_on:
       - agent-grok-image
 
+  # ==========================================================================
+  # Sandbox PG Image Builder (kitchen-sink postgres for parameterized dev DBs)
+  # Only pulled by the provisioner when a venture requests pg extensions; bare
+  # sandboxes stay on the light upstream postgres image. See
+  # docker/sandbox-pg.Dockerfile + docs/internal/specs/2026-07-13-sandbox-extensions-on-the-fly.md
+  # ==========================================================================
+  sandbox-pg-image:
+    build:
+      context: .
+      dockerfile: docker/sandbox-pg.Dockerfile
+    image: roboco-sandbox-pg
+    entrypoint: ["/bin/sh", "-c", "echo 'Sandbox PG (kitchen-sink) image built'"]
+    restart: "no"
+
   # ==========================================================================
   # Orchestrator - API Server + Agent Spawner
   # ==========================================================================