chore: add issue/PR templates, PR check, AGENTS.md and shared agent skills (#1043)

Adds a PR template (linked issue, what and why, how it was verified, review focus), a minimal reproduction field on the bug form and required acceptance criteria on the feature form, and CONTRIBUTING.md with the issue and PR rules. A PR check labels needs-info and leaves one comment when the issue link or the verification section is missing; it only reads the title and description.

AGENTS.md is now tracked, with CLAUDE.md linking to it, and covers layout, rules, commands, the checks CI runs, pitfalls and parallel worktrees. The verify and issue-fix skills are shared under .agents/skills and linked from .claude/skills; the rest of .claude/ stays local.

.github/pull_request_template.md

@@ -0,0 +1,17 @@
+Closes #
+
+## What and why
+
+<!-- 2–4 sentences: the change a user notices, and the root cause.
+     If it doesn't fit, the PR is probably too big; split it. -->
+
+## How it was verified
+
+<!-- Required. Commands you actually ran and their results; what else this could
+     affect and how you checked it still works.
+     UI changes: before/after screenshots. Multi-step interactions: a recording.
+     Backend changes: logs or command output. -->
+
+## Review focus
+
+<!-- Optional: anything you want reviewers to look at closely. -->