Add 'cratis arc screenplay' to fetch a .play file from a running Arc application

[woksin]

Problem / motivation

We are adding a Cratis.Arc.Screenplay package to the Arc repo that generates a Screenplay .play file describing an Arc application's full event model — concepts, modules, features, slices, commands, events, validation, authorization, constraints, queries, reactors and projections.

It surfaces the result the same way Arc already surfaces its other introspection data: a new endpoint

GET /.cratis/screenplay        → text/plain, the .play source

alongside the existing /.cratis/commands and /.cratis/queries.

What is missing is the CLI side. The point of generating a .play file is to get it into Cratis Studio, and today there is no command to fetch one, write it to disk, or import it.

Proposed solution

A cratis arc screenplay command under the existing arc branch, following the exact pattern of arc commands list / arc queries list (Source/Cli/Commands/Arc/):

cratis arc screenplay                              # print .play to stdout
cratis arc screenplay -o Library.play              # write to a file
cratis arc screenplay --url http://localhost:5100  # non-default app URL

It should subclass ArcCommand<T> so it inherits ArcSettings.ResolveUrl() (flag → ARC_URL env var → launch settings → default) and the standard "cannot connect" handling, then GET {url}/.cratis/screenplay and emit the body.

Note this endpoint returns text/plain, not JSON — unlike the existing introspection commands, the response is the artifact itself. So OutputFormatter's table/JSON shaping does not apply; the raw body is the output. Worth confirming how that should interact with the global -o/--output format flag, since -o is currently the format switch — an output file may need a different flag name (e.g. --file/-f) to avoid a collision.

Alternatives considered

• Generating the file in the CLI itself, by loading the app's assemblies offline. Rejected upstream: fluent IProjectionFor<T> and IConstraint definitions only materialise when Define() is actually executed, which needs a live DI container. The in-process endpoint gets full fidelity; an offline loader silently drops DI-dependent artifacts.
• Having Studio pull directly from the running app. Skips the CLI entirely, but loses the ability to diff a .play file in source control, and requires Studio to reach the developer's machine.

Additional context

Depends on the Arc-side endpoint landing first — that work is in progress in the Arc repo (Cratis.Arc.Screenplay). This issue can be picked up as soon as /.cratis/screenplay is available.

Acceptance criteria

• cratis arc screenplay fetches /.cratis/screenplay from the resolved URL and writes the .play source to stdout.
• A flag writes the output to a file instead, without mangling it (exact bytes, trailing newline preserved — Screenplay output ends with exactly one \n).
• URL resolution, connection-failure messaging and exit codes match the existing arc commands list behavior.
• Non-success HTTP status produces the standard server-error exit code and message.
• Carries [CliCommand], [CliExample] and [LlmDescription] attributes consistent with the sibling Arc commands.
• Specs in Source/Cli.Specs covering: successful fetch, write-to-file, server error, and connection failure.
• Documented under Documentation/arc/.

Follow-up (separate issue, not this one)

Importing the generated .play into Studio — either cratis arc screenplay --import or a dedicated cratis studio import command. Worth designing once the fetch path exists and we know what Studio's import contract looks like.