feat: interactive terminal — WS-bridged docker exec PTY

GET /services/:id/terminal upgrades to a WebSocket attached to a
`docker exec` PTY in the service's running replica. JSON frames both
ways: stdin/resize out, stdout/exit/error in. Shell probe tries each
candidate (bash→sh) since `command -v` needs a shell to run; distroless
images get a clean "no shell" error instead of an OCI failure dump.

- docker.Client gains ExecAttach + ExecResize wrappers
- Frontend: xterm.js replaces the one-off console section — live PTY,
  fit-addon resizes forwarded over the socket
- CLI: `containr shell <id>` bridges the local TTY (raw mode, SIGWINCH
  resize forwarding); `containr exec` keeps the one-off command path
- Access mirrors service logs: project manage scope required

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

docs/guides/CLI.md

@@ -86,6 +86,8 @@ operations                       live ops view: active deploys, queue depth,
                                24h failures, cron runs, backups
 activity                         enriched activity feed; --severity/--category/
                                --resource/--project/--limit filters
+shell <service-id>               interactive PTY shell inside the service
+                               container (bash→sh fallback; resizes follow)
 banners list|create|update|delete  announcement banners (admin);
                                active banners print on `auth status`
 invites list|create|revoke       team invite links (admin); create