docs: add feature request + improvement issue templates, auto-assign new issues (#21)

- Add 2-feature_request.md and 3-improvement.md templates (chooser now
  lists Bug report, Feature request, Improvement in order)
- Improve bug report template: cross-template pointer, Linux log hint,
  Related issues/PRs section, demo-DB reproduction hint
- Auto-assign every newly opened issue to the repo owner:
  assignees in template front matter + a workflows/auto-assign.yml
  (covers issues opened without a template)
- Add ISSUE_TEMPLATE/config.yml with Roadmap/README contact links
- Update README Contributing section

README.md

@@ -452,7 +452,7 @@ Highlights of what's next:
 
 Contributions, bug reports, and feature ideas are welcome. Gridline is Apache 2.0-licensed and intentionally stays open — no paywalled tiers, no bundled proprietary services.
 
-- Report a bug via the **[issue template](.github/ISSUE_TEMPLATE/bug_report.md)** — it walks you through environment details (OS, Gridline version, DB type/version, connection method) so we can reproduce issues quickly. Opening a [new issue](https://github.com/AdrianBonpin/gridline/issues/new) pre-fills the template automatically.
+- Open an issue via the **[template chooser](https://github.com/AdrianBonpin/gridline/issues/new)** — pick **Bug report** (with an environment table so we can reproduce issues quickly), **Feature request**, or **Improvement** (UX polish for existing features). New issues are auto-assigned to the maintainer.
 - Submit a pull request. Keep Tauri commands thin, type IPC boundaries explicitly, and follow the existing Rust/React conventions.
 
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