ci: add PR title conventional commit validation

[notgitika]

Description

Adds a GitHub Actions workflow that validates PR titles follow conventional commit format, aligned with the repo's commit conventions in AGENTS.md (feat:, fix:, chore:, docs:, etc.).

Key behaviors:

• Enforces lowercase subjects to match existing commit style
• Validates single-commit PRs match the PR title (prevents accidental bad squash-merge messages)
• Supports [WIP] prefix to skip validation on draft PRs
• Skips validation for bot and dependencies labeled PRs
• Uses pull_request_target for fork PR compatibility

Related Issue

N/A

Documentation PR

N/A

Type of Change

• New feature

Testing

• Manually verified workflow YAML syntax
• I ran npm run test:unit and npm run test:integ
• I ran npm run typecheck
• I ran npm run lint

Checklist

• I have read the CONTRIBUTING document
• My changes generate no new warnings
• Any dependent changes have been merged and published

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