feat: add --app flag support to slack create for linking existing apps

[srtaalej]

Changelog

slack create now accepts the --app flag alongside --template to scaffold a project and automatically link it to an existing app by fetching its manifest from App Settings.

Summary

This PR adds --app [ID] support to slack create. When used with --template, the CLI will:

1. Resolve an authenticated workspace that has access to the specified app
2. Fetch the app's manifest from the platform (apps.manifest.export)
3. Scaffold the project from the template (existing behavior)
4. Write the remote manifest to the project's manifest.json
5. Link the app to the project (defaults to local/dev unless the manifest uses slack-hosted runtime)

The --name flag takes precedence over the remote manifest's display name when both are provided.
Using --app without --template returns an error with guidance.

Example:

slack create my-project -t slack-samples/bolt-js-starter-template --app A0123456789

Preview

  📂 Project Create
     Cloning template slack-samples/bolt-js-starter-template
     To path ~/programming/slack-cli/my-project

  📦 Project Dependencies
     Added my-project/.slack
     Added my-project/.slack/.gitignore
     Added my-project/.slack/config.json
     Added my-project/.slack/hooks.json
     Updated app manifest source to "project" (local)
     Added package @slack/cli-hooks@1.3.2
     Installed dependencies using npm install

  📋 Next Steps
     Learn more about the project in the README.md
     Change into your project with cd my-project/
     Start developing and see changes in real-time with slack run

Testing

make test testdir=cmd/project testname=TestCreateCommand_AppFlag  # — 8 integration tests
make test testdir=cmd/project testname=Test_resolveAuthForApp  # — auth resolution unit tests
make test testdir=cmd/project testname=Test_writeManifestToProject  # — manifest write unit test
make test testdir=cmd/project testname=Test_linkAppToProject   # — app linking unit tests

Manual verification:

1. ./bin/slack create my-project -t slack-samples/bolt-js-starter-template --app <real-app-id>
2. Confirmed .slack/apps.dev.json contains linked app with correct team/app IDs
3. Confirmed manifest.json matches remote app's manifest
4. Confirmed --app without --template returns helpful error
5. Confirmed --name overrides manifest display name when provided

Notes

• Follow-up: manifest merging — Currently the remote manifest fully overwrites the template's
  manifest.json. A future PR will implement git-style merging where template and remote manifests
  are combined.

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

❌ Patch coverage is 65.88235% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.63%. Comparing base (c693443) to head (0d8bfd3).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/project/create_app.go	61.11%	16 Missing and 5 partials ⚠️
cmd/project/create.go	74.19%	4 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #565      +/-   ##
==========================================
- Coverage   71.66%   71.63%   -0.04%     
==========================================
  Files         226      227       +1     
  Lines       19115    19200      +85     
==========================================
+ Hits        13699    13754      +55     
- Misses       4209     4230      +21     
- Partials     1207     1216       +9     

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