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

.github/ISSUE_TEMPLATE/3-improvement.md

@@ -0,0 +1,40 @@
+---
+name: Improvement
+about: Suggest a refinement or UX polish for an existing Gridline feature
+title: "[Improvement]: "
+labels: ["enhancement"]
+assignees: ['AdrianBonpin']
+---
+
+<!--
+  For refinements to something that already exists — not brand-new features
+  (use the Feature request template for those) and not bugs (use Bug report).
+  Examples: a confusing label, a clunky interaction, a missing keyboard shortcut,
+  faster scroll, better default, accessibility fix.
+-->
+
+## Before you submit
+
+- [ ] I searched [existing issues](https://github.com/AdrianBonpin/gridline/issues?q=is%3Aissue) and this isn't a duplicate
+- [ ] This targets an **existing** feature (not a new one)
+- [ ] I will **not** paste secrets — redact any credentials as `<REDACTED>`
+
+## What's lacking today?
+
+<!-- The pain point in the current behavior. Point at the exact screen/flow. -->
+
+## Suggested change
+
+<!-- The refinement. What should it look like / behave like? Specific beats vague. -->
+
+## Why it matters
+
+<!-- Who benefits and how (time saved, fewer misclicks, consistency, accessibility, …). -->
+
+## Screenshots / recordings
+
+<!-- Current behavior, plus any mock of the desired behavior. -->
+
+## Related
+
+<!-- Links to related issues/PRs. -->