investigate_ci: show build errors for all failed jobs

[jstarks]

• Show build/job failure logs for all failed jobs, not just the first
• Always show build failures regardless of whether test artifacts exist
• Extract error-pattern lines (rustc, linker, etc.) from logs instead of dumping the raw tail
• Fall back to check-run annotations API when logs aren't available (e.g., run still in progress), so failures are visible immediately

[Copilot]

Pull request overview

This PR improves the CI failure investigation script to show build/job failure logs for all failed jobs instead of just the first one, and adds smarter error extraction from logs.

Changes:

• Iterate over all failed jobs (not just the first) and always show build failure logs regardless of test artifact availability
• Extract error-pattern lines (rustc, linker, etc.) from logs instead of dumping raw tail, with fallback to check-run annotations API
• Fix gh() helper to return empty string on failure when check=False (previously fell through to return stdout), and add quiet parameter

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