docs(contributing): search-first, linked issue, gates + Greptile bar (#7522)

## Thinking Path

> - Paperclip is the open source app people use to manage AI agents for
work
> - CONTRIBUTING.md and the PR template are the entry point for new
contributors and set quality expectations for every PR
> - Today they don't tell contributors to search for duplicate/related
work first, don't require a linked GitHub issue, and don't spell out the
merge bar (gates green + Greptile 5/5)
> - Letting duplicate or under-reviewed PRs land creates extra work for
reviewers and erodes quality
> - This PR updates CONTRIBUTING.md and the PR template to require a
search-first workflow, a linked issue, a clear merge bar, and explicit
encouragement to help other contributors land their PRs
> - The benefit is fewer duplicate PRs, easier triage, higher-quality
merges, and a kinder collaboration culture

## What Changed

- `CONTRIBUTING.md`: added a "Before You Start: Search First" section
directing contributors to search GitHub for duplicate PRs and related
issues, with a note that a fresh PR is okay if an old one is dead and
would be painful to merge.
- `CONTRIBUTING.md`: require every PR to link a GitHub issue in the
body, plus references to related/duplicate PRs and issues.
- `CONTRIBUTING.md`: added a section encouraging contributors to help
finish other people's PRs, thank collaborators, and be kind.
- `CONTRIBUTING.md`: tightened the merge bar — all Paperclip CI gates
must be green and Greptile must be 5/5 with no open P2s,
recommendations, or follow-ups.
- `CONTRIBUTING.md` + `.github/PULL_REQUEST_TEMPLATE.md`: updated the
thinking-path opener to "Paperclip is the open source app people use to
manage AI agents for work" (consistent across examples and template).
- `.github/PULL_REQUEST_TEMPLATE.md`: checklist gains items for
duplicate/related search, linked issue, gates green, and the Greptile
5/5 bar.

## Verification

- Docs-only change. Reviewed both files render correctly on GitHub.
- No code or tests affected; no CI behavior change beyond the new
checklist items contributors are asked to tick.

## Risks

- Low risk. Documentation and template only — no runtime impact.
Stricter contributor expectations may slow some PRs in the short term.

## Model Used

- Claude, `claude-opus-4-7` (extended thinking, tool use).

## Related

- Issue: #4188 — "Add issue templates and enforce issue-link check on
PRs" (the linked-issue requirement here is a step toward what #4188 asks
for, done in CONTRIBUTING/template rather than CI).
- Prior PR: #4260 — "docs: expand CONTRIBUTING.md and add issue
templates" (open; different scope — issue templates + broader expansion.
This PR is narrower and focused on the search-first + merge-bar
updates.)

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [x] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] I have searched GitHub for duplicate or related PRs and linked
them above
- [x] I have linked the GitHub issue this PR addresses (and any
related/duplicate issues)
- [x] I have run tests locally and they pass (docs-only change; no tests
to run)
- [x] I have added or updated tests where applicable (N/A — docs)
- [x] If this change affects the UI, I have included before/after
screenshots (N/A — docs)
- [x] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [ ] All Paperclip CI gates are green (will verify after push)
- [ ] Greptile is 5/5 with no open P2s, recommendations, or follow-ups
(pending review)
- [x] I will address all Greptile and reviewer comments before
requesting merge

.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