compose: add wrapper-first dogfood workspace (#1488)

Co-authored-by: marcie <[email protected]>

README.md

@@ -208,6 +208,50 @@ mosaic telemetry upload        # Dry-run unless opted in
 
 Consent state is persisted in config. Remote upload is a no-op until you run `mosaic telemetry opt-in`.
 
+## Standalone container deployment
+
+The `stack` profile runs PostgreSQL, Valkey, the gateway, and the bundled webUI. Copy
+`.env.example` to `.env`, generate `BETTER_AUTH_SECRET`, then start the profile:
+
+```bash
+cp .env.example .env
+printf 'BETTER_AUTH_SECRET=%s\n' "$(openssl rand -hex 32)" >> .env
+docker compose --profile stack up -d
+```
+
+The optional dogfood overlay gives one dedicated in-stack agent a writable stack
+worktree and its own read-only credential slot. It does not mount the fleet brain or
+any other seat. Prepare a `next`-based worktree and an unprivileged `stack-dogfood`
+seat outside the container, then set these paths in `.env`:
+
+```dotenv
+MOSAIC_DOGFOOD_WORKTREE=/path/to/mosaic-stack-worktrees/dogfood-1487
+MOSAIC_DOGFOOD_COMMON_GIT_DIR=/path/to/mosaic-stack/.git
+MOSAIC_DOGFOOD_SEAT_HOME=/path/to/.mosaic/fleet/agents/stack-dogfood
+```
+
+The common Git directory must match the worktree's `.git` pointer. The seat home
+must contain only that seat's credential at
+`secrets/gitea-mosaicstack-stack-dogfood.token`. Never place the token value in
+`.env`. Start the overlay with:
+
+```bash
+docker compose \
+  -f docker-compose.yml \
+  -f docker-compose.dogfood.yml \
+  --profile stack up -d
+```
+
+The overlay removes the general shell tool for every session, including admins.
+File tools stay inside the mounted checkout. Two dedicated delivery tools stage
+explicit paths, run the CI queue guard, push through `git-credential-mosaic`, and
+open PRs through `pr-create.sh`. They resolve only the `stack-dogfood` slot and fail
+if it is absent. The overlay enables Docker's init process so the R4 helper can
+establish the gateway's seat lineage below PID 1.
+
+This deployment route is separate from the local source-development restrictions
+below.
+
 ## Development
 
 ### Prerequisites