feat: support SLACK_CLI_APP_ICON_PATH env var for icon override

[srtaalej]

Summary

• Add SLACK_CLI_APP_ICON_PATH environment variable support to override the app icon path at install/deploy time
• Enables setting different icons per environment (dev/staging/prod) without changing project files
• Priority chain: SLACK_CLI_APP_ICON_PATH env var > manifest icon field > icon.png in project root
• Extracts shared resolveIconPath helper used by both Install() (hosted) and InstallLocalApp() (non-hosted) flows

Test plan

Unit tests

• go test ./internal/config/ -run Test_DotEnv — env var loading (set, empty, whitespace trimming)
• go test ./internal/pkg/apps/ -run Test_resolveIconPath — all 8 priority chain scenarios

Manual testing

From a Slack app project directory with assets/icon.png:

1. Baseline — default icon via slack run

   ./bin/slack run -e set-icon

   Confirm output shows Updated app icon: assets/icon.png

2. Override via env var

   export SLACK_CLI_APP_ICON_PATH=icon.png
   ./bin/slack run -e set-icon

   Confirm output shows Updated app icon: icon.png

3. Invalid env var path falls back

   export SLACK_CLI_APP_ICON_PATH=/nonexistent/icon.png
   ./bin/slack run -e set-icon

   Confirm warning about file not found and fallback to manifest/default icon

4. Unset env var restores default

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

   Confirm output shows Updated app icon: assets/icon.png

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 71.30%. Comparing base (783ac1a) to head (95ded37).

Files with missing lines	Patch %	Lines
internal/pkg/apps/install.go	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #519      +/-   ##
==========================================
+ Coverage   71.25%   71.30%   +0.04%     
==========================================
  Files         222      222              
  Lines       18682    18690       +8     
==========================================
+ Hits        13311    13326      +15     
+ Misses       4190     4184       -6     
+ Partials     1181     1180       -1     

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