feat(scripts): enhance buildkite-failures.ts to fetch and save full logs (#26177)

## Summary
- Fetches complete logs from BuildKite's public API (no token required)
- Saves logs to `/tmp/bun-build-{number}-{platform}-{step}.log`
- Shows log file path in output for each failed job
- Displays brief error summary (unique errors, max 5)
- Adds help text with usage examples (`--help`)
- Groups failures by type (build/test/other)
- Shows annotation counts with link to view full annotations
- Documents usage in CLAUDE.md

## Test plan
- [x] Tested with build #35051 (9 failed jobs)
- [x] Verified logs saved to `/tmp/bun-build-35051-*.log`
- [x] Verified error extraction and deduplication works
- [x] Verified `--help` flag shows usage

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Bot <claude-bot@bun.sh>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

CLAUDE.md

@@ -211,3 +211,24 @@ Built-in JavaScript modules use special syntax and are organized as:
 12. **Branch names must start with `claude/`** - This is a requirement for the CI to work.
 
 **ONLY** push up changes after running `bun bd test <file>` and ensuring your tests pass.
+
+## Debugging CI Failures
+
+Use `scripts/buildkite-failures.ts` to fetch and analyze CI build failures:
+
+```bash
+# View failures for current branch
+bun run scripts/buildkite-failures.ts
+
+# View failures for a specific build number
+bun run scripts/buildkite-failures.ts 35051
+
+# View failures for a GitHub PR
+bun run scripts/buildkite-failures.ts #26173
+bun run scripts/buildkite-failures.ts https://github.com/oven-sh/bun/pull/26173
+
+# Wait for build to complete (polls every 10s until pass/fail)
+bun run scripts/buildkite-failures.ts --wait
+```
+
+The script fetches logs from BuildKite's public API and saves complete logs to `/tmp/bun-build-{number}-{platform}-{step}.log`. It displays a summary of errors and the file path for each failed job. Use `--wait` to poll continuously until the build completes or fails.