fix(ci): heading-level mismatch in agent dianostic regex

[minhdqdev]

Summary

Fix a false-positive condition in the issue-triage workflow where all bug reports submitted via the GitHub form template were incorrectly flagged with state:triage-needed, even when fully compliant with the agent-first requirement.

Related Issue

Fixes #603

Changes

• Updated .github/workflows/issue-triage.yml to search for ### Agent Diagnostic (H3) instead of ## Agent Diagnostic (H2)
• GitHub form templates always render textarea field labels as H3 headings regardless of template configuration; the workflow regex was searching for H2, causing every form submission to be flagged as missing an agent diagnostic

Testing

The workflow now correctly recognizes the Agent Diagnostic section in form-submitted issues and only applies state:triage-needed when the section is actually empty or contains only the placeholder text.

Checklist

• Follows Conventional Commits format
• No breaking changes
• Related issue is referenced