docs(contributing): search-first, issue-link-or-describe, gate/Greptile bar, helping others

- Search GitHub for duplicate/in-flight PRs and related issues before starting.
- Drop the hard "issue required" gate: accept either `Fixes: #N` links OR an
  in-PR issue description following the new .github/ISSUE_TEMPLATE/ forms
  (bug, feature, adapter). Reviewers get the same fields either way.
- Keep CI gates + 5/5 Greptile (no open P2s, recommendations, or follow-ups)
  as merge requirements.
- Encourage helping other contributors' stalled PRs; preserve credit; be kind.
- PR template: add an explicit "Linked Issues or Issue Description" section
  and update the checklist to match the two-path rule.
- Thinking-path canonical opener: "Paperclip is the open source app people
  use to manage AI agents for work".

.github/PULL_REQUEST_TEMPLATE.md

@@ -7,13 +7,32 @@
   Aim for 5–8 steps. See CONTRIBUTING.md for full examples.
 -->
 
-> - Paperclip orchestrates AI agents for zero-human companies
+> - Paperclip is the open source app people use to manage AI agents for work
 > - [Which subsystem or capability is involved]
 > - [What problem or gap exists]
 > - [Why it needs to be addressed]
 > - This pull request ...
 > - The benefit is ...
 
+## Linked Issues or Issue Description
+
+<!--
+  Required. Pick ONE of the following two paths:
+
+  (A) Issue exists — tag each linked issue with `Fixes: #123`, `Closes #123`,
+      or `Refs #123`. Include duplicates and closely related issues too.
+
+  (B) No issue exists — describe the underlying problem here, following the
+      relevant issue template so reviewers get the same fields:
+        • Bug:     .github/ISSUE_TEMPLATE/bug_report.yml
+        • Feature: .github/ISSUE_TEMPLATE/feature_request.yml
+        • Adapter: .github/ISSUE_TEMPLATE/adapter_request.yml
+
+  See CONTRIBUTING.md → "Link Issues or Describe Them In-PR".
+-->
+
+-
+
 ## What Changed
 
 <!-- Bullet list of concrete changes. One bullet per logical unit. -->
@@ -60,9 +79,13 @@
 - [ ] I have included a thinking path that traces from project context to this change
 - [ ] I have specified the model used (with version and capability details)
 - [ ] I have checked ROADMAP.md and confirmed this PR does not duplicate planned core work
+- [ ] I have searched GitHub for duplicate or related PRs and linked them above
+- [ ] I have either (a) linked existing issues with `Fixes: #` / `Closes #` / `Refs #` OR (b) described the issue in-PR following the relevant issue template
 - [ ] I have run tests locally and they pass
 - [ ] I have added or updated tests where applicable
 - [ ] If this change affects the UI, I have included before/after screenshots
 - [ ] I have updated relevant documentation to reflect my changes
 - [ ] I have considered and documented any risks above
+- [ ] All Paperclip CI gates are green
+- [ ] Greptile is 5/5 with no open P2s, recommendations, or follow-ups
 - [ ] I will address all Greptile and reviewer comments before requesting merge