feat: add --manifest-source flag to run and deploy commands

[srtaalej]

Summary

• Adds a --manifest-source=project|remote flag to slack run and slack deploy commands
• When manifest sync detects differences during run/deploy, this flag allows non-interactive resolution without requiring slack manifest sync to be run separately
• Updates the non-TTY error remediation to reference --manifest-source instead of --force/--force-remote (which are only available on manifest sync)

Closes #628

Test plan

• make lint passes
• make test passes
• Manual test: slack run --manifest-source=project pushes local manifest without prompting
• Manual test: slack run --manifest-source=remote pulls app settings without prompting
• Manual test: slack deploy --manifest-source=project works in non-TTY (CI) environments
• Manual test: slack run --manifest-source=invalid returns a clear validation error

[codecov]

Codecov Report

❌ Patch coverage is 81.81818% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.27%. Comparing base (b2ad402) to head (b021d22).

Files with missing lines	Patch %	Lines
cmd/platform/deploy.go	33.33%	2 Missing ⚠️
cmd/platform/run.go	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #630      +/-   ##
==========================================
+ Coverage   72.24%   72.27%   +0.02%     
==========================================
  Files         238      238              
  Lines       20072    20088      +16     
==========================================
+ Hits        14502    14518      +16     
- Misses       4299     4300       +1     
+ Partials     1271     1270       -1     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.