Phase 20: per-container inspect + start/stop/restart, local + agent (0.26.0)

Stack Overview now renders each service as an expandable ContainerCard with a
curated single-container inspect view and admin start/stop/restart buttons,
both for local stacks (GET/POST /api/containers/{id}[/{action}]) and remote
stacks (proxied via /api/agents/{id}/containers/* to the agent's new
/agent/containers/* endpoints). Only compose-managed containers are exposed.

Also bumps version 0.23.0 -> 0.26.0 (the bumps for the already-committed
Phase 18 image-prune / Phase 19 compose-validate were missed) and backfills
README sections for Phase 18/19/20.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.23.0",
+  "version": "0.26.0",
   "type": "module",
   "scripts": {
     "dev": "vite",