chore: add Go version lint check, modernize issue templates (#557)

- Add go.mod version check in lint workflow (fails if go directive != 1.20)
- Add golang.org/x/sys to dependabot ignore list (newer versions require Go 1.21+)
- Fix golang.org/x/text ignore comment
- Migrate issue templates from markdown to YAML forms:
- Update PR template (dev → main, add go.mod checklist item)
- Update .gitignore for YAML issue templates

.github/dependabot.yml

@@ -9,7 +9,8 @@ updates:
     ignore:
       - dependency-name: "modernc.org/sqlite"
         versions: [">=1.32.0"]  # v1.32+ requires Go 1.21, project is pinned to Go 1.20
-      - dependency-name: "golang.org/x/text"  # will be removed in upcoming refactoring
+      - dependency-name: "golang.org/x/text"  # indirect dep, newer versions may require Go 1.21+
+      - dependency-name: "golang.org/x/sys"  # newer versions require Go 1.21+
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule: