feat(create): fetch remote manifest after linking app during create

[srtaalej]

Changelog

slack create --app now fetches the app's manifest from remote settings and writes it to the project's manifest.json, replacing the template's generic manifest with the actual app configuration.

Summary

Follow-up to #565. After linking an existing app during slack create --app --template, the CLI now fetches the remote manifest from App Settings and overwrites the template's manifest.json with it.

This also updates LinkExistingApp to return (*types.SlackAuth, error) so callers can use the auth token for subsequent API calls (like manifest fetching).

Design decisions:

• Manifest source remains local in .slack/config.json — the remote content is written locally
• Only manifest.json is overwritten (not manifest.ts/.js which are code files in Deno projects)
• On fetch failure, a warning is printed and the command continues — the project and link still succeed

Testing

make test testdir=cmd/project testname=TestCreateCommand_AppFlag_FetchesRemoteManifest
make test testdir=cmd/project testname=TestCreateCommand_AppFlag
make test testdir=cmd/project testname=Test_Project_InitCommand
make test testdir=cmd/app testname=TestLinkCommand

Manual:

1. make build
2. ./bin/slack create my-test -t slack-samples/bolt-js-starter-template --app <real-app-id> --environment local
3. Verify my-test/manifest.json contains the remote app's manifest
4. Test with an invalid app ID to confirm the warning is shown and the project is still created

Notes 🔴

• Open question: should we set manifest.source to remote in .slack/config.json after writing the fetched manifest? Currently left as local so the user "owns" the file going forward.

A future PR could add merging logic between template and remote manifests.

• manifest.ts/.js (Deno SDK) projects are not supported

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 76.47059% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.70%. Comparing base (e37243b) to head (6d6749a).

Files with missing lines	Patch %	Lines
cmd/project/create.go	80.48%	4 Missing and 4 partials ⚠️
cmd/app/link.go	55.55%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #585      +/-   ##
==========================================
+ Coverage   71.64%   71.70%   +0.06%     
==========================================
  Files         226      226              
  Lines       19148    19189      +41     
==========================================
+ Hits        13718    13759      +41     
+ Misses       4221     4219       -2     
- Partials     1209     1211       +2     

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