build: rewrite issue templates

Problem: issue templates used upstream stevearc references, severity
dropdowns, outdated lazy.nvim bootstrap, and the .yml extension
inconsistent with other repos.

Solution: replace with .yaml templates matching cp.nvim/diffs.nvim
style. Bug report uses prerequisites checkboxes, checkhealth output,
modern lazy.nvim bootstrap with vim.g.oil pattern. Feature request
uses problem/solution/alternatives format. Add config.yaml to disable
blank issues and link discussions.

.github/ISSUE_TEMPLATE/config.yaml

@@ -0,0 +1,5 @@
+blank_issues_enabled: false
+contact_links:
+  - name: Questions
+    url: https://github.com/barrettruth/oil.nvim/discussions
+    about: Ask questions and discuss ideas