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>

backend/main.py

@@ -18,6 +18,7 @@ from routers import (
     audit,
     auth,
     backups,
+    containers,
     destinations,
     editor,
     files,
@@ -55,7 +56,7 @@ async def lifespan(app: FastAPI):
     schedule_task.cancel()
 
 
-app = FastAPI(title="StackPilot", version="0.23.0", lifespan=lifespan)
+app = FastAPI(title="StackPilot", version="0.26.0", lifespan=lifespan)
 
 app.add_middleware(
     CORSMiddleware,
@@ -76,6 +77,7 @@ async def docker_error_handler(_request: Request, exc: DockerError):
 
 app.include_router(auth.router)
 app.include_router(stacks.router)
+app.include_router(containers.router)
 app.include_router(system.router)
 app.include_router(volumes.router)
 app.include_router(editor.router)