feat: add slack manifest diff command

[mwbrooks]

Changelog

Added the slack manifest diff command, which prints differences between the project manifest and the app settings.

Summary

This pull request adds a read-only slack manifest diff command that compares the project manifest against the app settings on Slack and prints any differences.

• Useful for CI/CD guardrails, pre-check before deploys, and debugging manifest conflicts.
• Read-only - no API mutations, no file writes, no prompts.
• Carved out from PR feat: add two-way manifest sync between project and app settings #543 to make that PR easier to review. The two-way slack manifest sync command will build on these primitives.
• Ships without an experiment gate because it's a stand-alone read-only command. Happy to gate it, if reviewers want.
• Exits 0 even when differences are found, to match slack manifest validate (which exits 0 even when the manifest is invalid). Standardizing exit-code semantics for scripting use-cases is a separate, project-wide conversation that should change validate and diff together.

Preview

$ slack manifest --help

SUBCOMMANDS
  diff        Show differences between the project manifest and app settings
  info        Print the app manifest of a project or app
  validate    Validate the app manifest generated by a project

$ slack manifest diff --help

Compare the project manifest with app settings and print any differences.

USAGE
  $ slack manifest diff [flags]

Testing

Outside of a project will error:

$ ./bin/slack manifest diff

# Expect:
# → This is an invalid Slack app project directory (invalid_app_directory)

Create a project:

$ ./bin/slack create my-app/
# → Select any project type

$ cd my-app/

Inside of a project with no App IDs will error:

$ ../bin/slack manifest diff

# Expect:
# →  A valid installation of this app is required to take this action (installation_required)

Create an App ID:

$ ../bin/slack manifest diff
# → Environment: "local"
# → Team: Any

Compare Project to App Settings Manifest:

$ ../bin/slack manifest diff

📚 App Manifest
   Project manifest and app settings are in sync

• Run slack manifest diff inside a project where the local manifest matches app settings; verify the "Project manifest and app settings are in sync" message appears.
• Edit the project manifest (e.g., change display_information.name) without deploying. Run slack manifest diff and verify the changed field is listed with both the project value and the app settings value.
• Add a brand-new field to the project manifest. Run slack manifest diff and verify it shows up under "(only in project)".
• Run slack manifest --help and verify diff appears alongside info and validate.
• Run slack manifest diff --app <app-id> --token <token> (or with SLACK_AUTH_TOKEN set) and verify the command runs end-to-end with no interactive prompts — the same diff output is produced as the interactive case.

Notes

• Display output uses the existing style package formatting carried over from the sync branch. If reviewers prefer a leaner format (e.g., one line per diff), happy to simplify in a follow-up.

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 66.97674% with 71 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.63%. Comparing base (d7dfda7) to head (413f272).

Files with missing lines	Patch %	Lines
internal/manifest/display.go	10.41%	42 Missing and 1 partial ⚠️
internal/manifest/diff.go	83.50%	8 Missing and 8 partials ⚠️
cmd/manifest/diff.go	79.48%	4 Missing and 4 partials ⚠️
internal/manifest/flatten.go	86.66%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #591      +/-   ##
==========================================
- Coverage   71.67%   71.63%   -0.04%     
==========================================
  Files         226      230       +4     
  Lines       19176    19391     +215     
==========================================
+ Hits        13744    13891     +147     
- Misses       4221     4275      +54     
- Partials     1211     1225      +14     

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