chore: require issue link on all PRs

- PR template: move "Closes #" to top as required field with explicit
  warning that PRs without a linked issue are closed without review
- CONTRIBUTING.md: add mandatory issue-first policy with clear rationale
- Add require-issue-link.yml workflow: checks PR body for a closing
  keyword (Closes/Fixes/Resolves #NNN) on open/edit/reopen/sync events;
  posts a comment and fails CI if no reference is found

PR body is bound to an env var before shell use (injection-safe).
The github-script step uses the API SDK, not shell interpolation.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/pull_request_template.md

@@ -1,3 +1,10 @@
+## Linked Issue
+
+> **Required.** PRs without a linked issue are closed without review.
+> Open an issue first if one doesn't exist: https://github.com/gsd-build/get-shit-done/issues/new/choose
+
+Closes #
+
 ## What
 
 <!-- One sentence: what does this PR do? -->
@@ -6,8 +13,6 @@
 
 <!-- One sentence: why is this change needed? -->
 
-Closes #<!-- issue number -->
-
 ## How
 
 <!-- Brief description of the approach taken. Skip for trivial changes. -->
@@ -35,6 +40,7 @@ Closes #<!-- issue number -->
 
 ## Checklist
 
+- [ ] Issue linked above (`Closes #NNN`) — **PR will be auto-closed if missing**
 - [ ] Follows GSD style (no enterprise patterns, no filler)
 - [ ] Updates CHANGELOG.md for user-facing changes
 - [ ] No unnecessary dependencies added