feat(build): make summer new apps admin-ready

- NewOptions.AdminSecret: summer new writes SUMMER_ADMIN__JWT__SECRET
  to .env (still 0600), generated from 32 crypto/rand bytes when empty
  and validated as at least 32 .env-safe characters
- .env.example gets an empty SUMMER_ADMIN__JWT__SECRET= placeholder
  with a comment; the secret never reaches the printed next steps
- The next steps and the generated CLAUDE.md explain the admin at
  /backend: make:admin-controller, build, migrate and
  summer admin:create --email you@example.com --superuser
- Goldens use a fixed, non-secret admin secret fixture
- Scaffolding, installation and users-and-permissions docs updated

internal/build/testdata/new-no-plugin/CLAUDE.md.golden

@@ -23,6 +23,8 @@ summer migrate                     # run plugin migrations
 summer serve                       # start the HTTP server
 summer make:plugin golem15.<name>  # scaffold a plugin module under plugins/<name>
 summer plugin:add plugins/<name>   # register it in summer.yaml, go.mod and go.work
+summer make:admin-controller golem15.<plugin> <Name>  # scaffold an admin controller with its YAML
+summer admin:create --email you@example.com --superuser  # create a backend administrator (password prompted)
 ```
 
 ## Layout
@@ -30,13 +32,23 @@ summer plugin:add plugins/<name>   # register it in summer.yaml, go.mod and go.w
 - `summer.yaml`: the application manifest (module path, binary name, ordered plugin list).
 - `plugins/`: local plugin modules.
 - `config/`: YAML configuration (`app.yaml`, `database.yaml`, `http.yaml`, `storage.yaml`). Any key can be overridden with a `SUMMER_` environment variable, a double underscore separating path segments (`SUMMER_DATABASE__DSN` sets `database.dsn`).
-- `.env`: local secrets (database DSN, application key). Never commit it; `.gitignore` excludes it. `.env.example` lists the variables.
+- `.env`: local secrets (database DSN, application key, admin token secret). Never commit it; `.gitignore` excludes it. `.env.example` lists the variables.
 - `compose.yaml`: the local PostgreSQL 16.
 - `main.go`, `plugins.gen.go`: generated by `summer build` from `summer.yaml`. Do not edit them by hand; they are committed.
 - `go.work`: the workspace that joins the application and its plugin modules.
 
 Plugins are Go modules compiled into the binary at build time; there is no runtime plugin loading. Add a plugin with `summer make:plugin` and `summer plugin:add`, then run `summer build`.
 
+## Admin
+
+The backend admin (the admin SPA and its JSON API) lives under `/backend`, at `http://127.0.0.1:8080/backend` with the default `summer serve` address. It is mounted only when a plugin registers an admin controller; until then `summer serve` prints `admin: off`. To add it:
+
+1. `summer make:admin-controller golem15.<plugin> <Name>` scaffolds the controller and its YAML. The generated controller requires the permission `golem15.<plugin>.access_<name>` (`<name>` is the controller name in snake case): declare it in the plugin's `Permissions()` (`pact.HasPermissions`) or the admin refuses to start. Its `NewRecord` returns `nil` until you return the plugin's model.
+2. `summer build`, then `summer migrate`.
+3. `summer admin:create --email you@example.com --superuser` creates the first administrator; the password is prompted.
+
+`.env` already holds `SUMMER_ADMIN__JWT__SECRET`, which signs admin tokens.
+
 ## Naming convention
 
 - `sm-<name>-app`: an application, like this repository.

internal/build/testdata/new-no-plugin/next-steps.txt.golden

@@ -8,3 +8,10 @@ Next steps, inside sm-demo-app:
 
 The app has no plugins yet. Add one with summer make:plugin golem15.<name>
 and summer plugin:add plugins/<name>, then run summer build again.
+
+Admin, at http://127.0.0.1:8080/backend once a plugin registers an admin
+controller (.env already holds SUMMER_ADMIN__JWT__SECRET):
+  summer make:admin-controller golem15.<plugin> <Name>
+  summer build
+  summer migrate
+  summer admin:create --email you@example.com --superuser