ci: Fix several robustness issues in claude-review workflow

ci: Fix several robustness issues in claude-review workflow

- Use github.paginate() for listComments to handle PRs with 100+ comments
- Make line optional in review schema to allow file-level comments
- Skip createReviewComment for comments without a line number
- Fix failed count to exclude skipped file-level comments
- Pass review result via env var instead of expression injection
- Use core.warning() instead of console.log() for JSON parse failures
- Fix MARKER insertion for single-line summaries that have no newline
- Require "@claude review" instead of just "@claude" to trigger

Co-developed-by: Claude <claude@anthropic.com>