docs: add pr-title skill guidance

Co-authored-by: chatgpt-codex-connector[bot] <199175422+chatgpt-codex-connector[bot]@users.noreply.github.com>
Assisted-by: openai/gpt-5.3-codex on opencode

Author: scarf005

.agents/skills/pr-title/SKILL.md

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+---
+name: pr-title
+description: Chooses and validates the best PR title type and optional scope using changed files. Enforces docs-heavy PRs to use docs type and refreshes semantic scopes when new mods are added.
+---
+
+# PR Title
+
+Use this skill when drafting or reviewing pull request titles.
+
+## Goal
+
+- Choose an appropriate Conventional Commit type.
+- Choose an optional scope only when it clearly improves precision.
+- Prevent misleading types (for example `feat` for docs-only work).
+
+## Rules
+
+1. PR title format must be `<type>: <subject>` or `<type>(<scope>): <subject>`.
+2. Type must follow project changelog categories in `docs/en/contribute/changelog_guidelines.md`.
+3. If changes are mostly under `docs/` (>80% of changed files), type must be `docs`.
+4. Scope is optional. Use it only when specific and accurate (`UI`, `i18n`, `balance`, `port`, `lua`, `mods`, `mods/<MOD_ID>`).
+5. Do not use `feat` for documentation-only PRs.
+
+## Workflow
+
+1. Gather changed files for the branch against `upstream/main`.
+2. Propose a PR title with best type and optional scope.
+3. Validate it with:
+
+```sh
+deno task pr-title --title "<type>(<scope>): <subject>"
+```
+
+4. If validation fails, adjust type/scope and rerun.
+
+## Semantic Scope Refresh
+
+When new mods are added (`data/mods/<MOD_ID>/modinfo.json` as added files), this skill must ensure semantic scopes are refreshed.
+
+- The validator handles this automatically by running `deno task semantic`.
+- Commit any resulting `.github/semantic.yml` update together with the PR-title changes.