chore: add PR title validation skill and task

[scarf005]

Purpose of change (The Why)

Improve PR title quality and changelog consistency by adding a reusable skill that picks proper Conventional Commit type/scope and enforces docs-heavy PRs to use docs type.

Describe the solution (The How)

• add .agents/skills/pr-title/SKILL.md with explicit PR type/scope selection rules
• add skill-local script .agents/skills/pr-title/scripts/pr_title.ts to validate <type>(<scope>): <subject> and enforce docs-majority rule (>80% in docs/ => docs type)
• follow Agent Skills scripts/ structure instead of adding a project-level deno task for skill-only behavior
• auto-run deno task semantic when newly added mods are detected

Describe alternatives you've considered

• relying only on reviewer judgment (rejected: inconsistent and error-prone)
• adding a static lint-only rule without file-aware checks (rejected: cannot enforce docs-majority policy)

Testing

• deno fmt .agents/skills/pr-title/SKILL.md .agents/skills/pr-title/scripts/pr_title.ts deno.jsonc
• deno check .agents/skills/pr-title/scripts/pr_title.ts
• deno lint .agents/skills/pr-title/scripts/pr_title.ts .agents/skills/pr-title/SKILL.md
• deno run -R -N --allow-run=git,deno .agents/skills/pr-title/scripts/pr_title.ts --title \"docs: add pr-title skill guidance\"

Additional context

None.

Checklist

Mandatory

• I wrote the PR title in conventional commit format.
• I ran the code formatter.
• I linked any relevant issues using github keyword syntax like closes #1234 in Summary of the PR so it can be closed automatically.
• I've committed my changes to new branch that isn't main so it won't cause conflict when updating main branch later.