feat: add retry composite action with exponential backoff

[castrojo]

Summary

Automation audit Phase 3: reusable retry-with-backoff composite action.

Addresses FM1 (no retry on transient failures) and FM2 (no backoff) from the automation audit.

Usage:

- name: Push with retry
  uses: projectbluefin/actions/actions/retry@<sha>
  with:
    command: buildah push "$IMAGE" "docker://$REGISTRY/$IMAGE:$TAG"
    max_attempts: 3
    initial_wait_seconds: 10
    retry_on: "rate limit|timeout|429|503|connection reset"

Features:

• Configurable max attempts (default: 3)
• Exponential backoff (wait doubles each retry)
• Optional error-pattern filtering (only retry on specific errors)
• Reports attempt count and success/fail as outputs

Closes #583 (Phase 3 of automation audit).

Assisted-by: Claude Sonnet 4.5 via pi

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

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