ci: fix duplicate CI runs by limiting push triggers to main branch

[blink-so]

Problem

Currently, CI runs twice for feature branches with pull requests:

1. Push event: When pushing commits to feature branch
2. Pull request event: When opening/updating the PR

This wastes CI resources and creates duplicate status checks.

Solution

Updated the GitHub Actions workflow triggers to follow best practices:

Before:

on:
  push:
  pull_request:

After:

on:
  push:
    branches: [ main ]
  pull_request:
    branches: [ main ]

Behavior Changes

Scenario	Before	After
Push to feature branch	✅ CI runs	❌ No CI run
Open PR from feature branch	✅ CI runs	✅ CI runs
Push to main branch	✅ CI runs	✅ CI runs
Update PR (push to feature branch)	✅ CI runs twice	✅ CI runs once

Benefits

• Eliminates duplicate CI runs for feature branches with PRs
• Saves CI resources and reduces build queue times
• Follows GitHub Actions best practices
• Maintains full test coverage - all PRs are still tested
• Preserves main branch protection - pushes to main still trigger CI

Testing

This PR itself will demonstrate the fix:

• ✅ No CI run when pushing the feature branch
• ✅ CI runs once when the PR is opened
• ✅ No duplicate runs