Phase 1: Add concurrency control to workflows

[RAprogramm]

Task

Add concurrency groups to GitHub Actions workflows to prevent redundant CI runs and save resources.

Implementation

# ci.yml and reusable-ci.yml
concurrency:
  group: ${{ github.workflow }}-${{ github.ref }}
  cancel-in-progress: ${{ github.event_name == 'pull_request' }}

Benefits

• Cancel outdated CI runs when new commits are pushed to PR
• Save CI minutes
• Faster feedback loop

Acceptance Criteria

• Concurrency added to ci.yml
• Concurrency added to reusable-ci.yml
• Tested: pushing multiple commits to PR cancels old runs
• No impact on main branch builds

Parent: #175