Evaluate removing superdeck_cli in favor of documented builder setup

## Summary
Evaluate whether SuperDeck should remove `superdeck_cli` and rely on documented `superdeck_builder` setup instead. This is intentionally deferred from the plugin PR so that PR can stay focused on the Mermaid/PDF plugin work.

## Context
The CLI currently wraps deck build/watch, setup guidance, pubspec asset patching, macOS entitlement patching, and build plugin registration. The runtime itself does not need the CLI; generated deck output and generated assets live under `.superdeck/`.

## Questions to resolve
- Can `superdeck_builder` be the primary supported build surface without a CLI wrapper?
- Where should manual setup docs live for `pubspec.yaml` assets and macOS entitlements?
- Should the demo include a small reusable `tool/build_deck.dart` template for direct builder usage?
- What replaces current CI/Firebase/demo commands that run `dart run superdeck_cli:main build`?

## Setup details to document
- Add `.superdeck/` to `flutter.assets` in `pubspec.yaml`.
- Keep macOS entitlements documented for debug/release where local file access, network client/server, downloads, and JIT are needed.
- Explain that build plugins such as Mermaid generate local files under `.superdeck/` and the Flutter app bundles those files for release/web builds.

## Acceptance criteria
- A decision on whether to remove, shrink, or keep `superdeck_cli`.
- A replacement build/watch workflow for demo, CI, docs, and plugin authoring.
- Documentation for manual setup, including pubspec assets and macOS entitlements.

Start by tracing the current superdeck_cli build/watch workflow and the documented superdeck_builder setup, including pubspec.yaml assets and macOS entitlements. Compare the demo, CI, Firebase, and plugin-authoring commands with the proposed tool/build_deck.dart template. Done means a decision on the CLI, a replacement workflow, and complete manual setup documentation.

Written by the indexing model from the issue text.