Add a start flag, and let --check survive a first deploy

-e start=false (mode sync in CI) lands the payload and starts nothing, for
migrations. A check run on a never-deployed stack now skips Compose with a note
instead of failing on the missing directory.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/ci-github-actions.md

@@ -67,7 +67,7 @@ Actions → **deploy** → Run workflow. Three fields:
 | Field | Values | Meaning |
 | --- | --- | --- |
 | `stack` | the dropdown | Which playbook to run. `site` is all of them. |
-| `mode` | `check`, `deploy` | `check` runs `--check --diff` and touches nothing |
+| `mode` | `check`, `sync`, `deploy` | `check` runs `--check --diff` and touches nothing; `sync` copies the files for real but starts no container (`-e start=false`) |
 | `pull` | `policy`, `always`, `missing`, `never` | `always` fetches newer images **and** enables per-stack update work |
 
 Work up from harmless to real the first time: