feat(cmd): auto-sync manifest from remote when creating with --app

[srtaalej]

Changelog

slack create --app now automatically syncs the manifest from app settings to the local project.

Summary

The CLI handoff from app settings to local development currently requires two commands: slack create --app ... --template ... followed by slack manifest sync. This PR consolidates them — when --app is passed to slack create, the remote manifest is fetched from app settings and written to the local project after linking.

Changes:

• LinkExistingApp now returns the auth token so callers can use it for follow-up API calls
• After linking an app during create, the remote manifest is fetched via GetManifestRemote and written locally via WriteManifestLocal
• If the fetch or write fails, a warning is shown with instructions to run slack manifest sync --manifest-source=remote manually

Preview

No UI changes — output is the same as before, minus the need for a separate slack manifest sync step.

Testing

1. Go to https://api.slack.com/apps and create a new app through the "Create New App" flow
2. Note the App ID and Team ID from the app settings page
3. Build the CLI: make build
4. Run:

   ./bin/slack create --template slack-samples/bolt-js-starter-template --app <APP_ID> --name "test-create-sync" --team <TEAM_ID> --environment local
   

5. cd into the created project and run ./bin/slack run
6. Verify there is no manifest sync prompt — the project manifest should already match app settings

Notes

• The manifest fetch/write error is non-blocking — if it fails, the project is still usable and the user can run slack manifest sync --manifest-source=remote manually
• manifest.Sync is unchanged — the create flow inlines only the fetch and write steps since it always uses remote as the source of truth

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 66.66667% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.19%. Comparing base (fcb0782) to head (944e498).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/project/create.go	55.55%	4 Missing ⚠️
cmd/app/link.go	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #677      +/-   ##
==========================================
- Coverage   78.19%   78.19%   -0.01%     
==========================================
  Files         239      239              
  Lines       18132    18139       +7     
==========================================
+ Hits        14178    14183       +5     
- Misses       3954     3956       +2     

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