feat: support icon path in project config.json

[srtaalej]

Summary

• Adds an icon field to .slack/config.json so users can specify an icon path without modifying their app manifest
• Icon resolution order: manifest icon field > config.json icon field > icon.png in project root
• Adds GetIconPath / SetIconPath to ProjectConfigManager interface following the same patterns as GetProjectID / SetProjectID
• Migrates existing tests in add_test.go and install_test.go to use AddDefaultMocks() instead of manually duplicating mock setup for ProjectConfigMock

Example config.json

{
  "icon": "assets/icon.png",
  "project_id": "abc-123"
}

Test plan

Unit tests

go test ./internal/config/ -run Test_ProjectConfig_GetIconPath -v   # 3 cases
go test ./internal/config/ -run Test_ProjectConfig_SetIconPath -v   # 3 cases
go test ./...                                                       # full suite

Manual testing

1. Build the branch

git fetch origin ale-icon-config
git checkout ale-icon-config
go build -o bin/slack .

2. Set up a test app (or use an existing bolt app)

./bin/slack create
cd <app-name>

3. Add an icon to config.json

Place an image file somewhere in the project (e.g. assets/icon.png), then add the path to .slack/config.json:

{
  "icon": "assets/icon.png",
  "project_id": "..."
}

4. Run with the set-icon experiment

../bin/slack run -e set-icon

Verify you see Updated app icon: assets/icon.png in the output.

5. Verify manifest takes precedence over config.json

Set icon in both the manifest (slack.yaml / manifest.ts) and config.json with different paths. Run again — the manifest path should be used.

6. Verify fallback to project root

Remove icon from both manifest and config.json. Place icon.png in the project root. Run again — should pick up icon.png.

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.27%. Comparing base (1a7c949) to head (54bc2bb).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/pkg/apps/install.go	0.00%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #514      +/-   ##
==========================================
+ Coverage   71.17%   71.27%   +0.09%     
==========================================
  Files         222      222              
  Lines       18678    18704      +26     
==========================================
+ Hits        13294    13331      +37     
+ Misses       4201     4191      -10     
+ Partials     1183     1182       -1     

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