Phase 23: per-stack secrets & configs (compose file-based), local + agent (0.29.0)

Manage Docker secrets and configs per stack from a new Secrets tab on Stack/
RemoteStackDetail. Content is stored as files inside the stack dir
(.secrets/<name>, .configs/<name>; dir 0700 / file 0600) and referenced from the
compose file with relative `file:` paths, so the daemon reads them without any
HOST_ROOT_PREFIX dependency. Content is write-only — the API only ever returns
metadata (name, kind, size).

- secret_service: write/delete/list (metadata only)/exists/rel_path/attach/detach;
  name validation rejects traversal/hidden/separators, content capped at 1 MiB.
- compose_edit_service: add/remove secret and config (top-level defs pruned when
  no service still references them).
- routers/secrets.py (admin-only, audit secret.*) + agent endpoints + multi-host
  proxy (audit agent.secret.*).
- Frontend SecretsPanel (create/list/delete + per-row attach/detach to a service;
  config rows take a mount target), agentId-aware for remote stacks.

Verified: name-sandbox + perms + metadata-only listing unit-tested; compose
add/remove round-trips to clean YAML; py_compile + backend/agent/frontend image
builds + route smoke-test (local/agent/proxy). Live exec check (/run/secrets/<name>
on a deployed stack) and swarm path are hardware-verify debt (swarm dropped: A).

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

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.28.0",
+  "version": "0.29.0",
   "type": "module",
   "scripts": {
     "dev": "vite",