Update CONTRIBUTING.md to require PR template, Greptile 5/5, and tests (#2618)

## Thinking Path

> - Paperclip orchestrates AI agents for zero-human companies
> - Contributors submit pull requests to improve the codebase
> - We have a PR template at `.github/PULL_REQUEST_TEMPLATE.md` that
standardizes PR descriptions
> - But PRs created via the API or other tooling sometimes bypass the
template
> - We also require Greptile automated review and passing tests, but
this wasn't clearly documented
> - This PR updates CONTRIBUTING.md to explicitly require use of the PR
template, a 5/5 Greptile score, and passing tests
> - The benefit is contributors have clear, upfront expectations for
what a mergeable PR looks like

## What Changed

- Added a new "PR Requirements (all PRs)" section to CONTRIBUTING.md
with three subsections:
- **Use the PR Template** — links to `.github/PULL_REQUEST_TEMPLATE.md`
and explains it must be used even when creating PRs outside the GitHub
UI
  - **Tests Must Pass** — requires local test runs and green CI
  - **Greptile Review** — requires 5/5 score with all comments addressed
- Updated Path 1 and Path 2 bullet points to reference the PR template,
Greptile 5/5, and CI requirements specifically
- Updated "Writing a Good PR message" section to link to the PR template
and clarify all sections are required

## Verification

- Read the updated CONTRIBUTING.md and verify it clearly references the
PR template, Greptile 5/5 requirement, and test requirements
- Verify all links to `.github/PULL_REQUEST_TEMPLATE.md` resolve
correctly

## Risks

- Low risk — documentation-only change, no code affected

## Model Used

- Provider: Anthropic Claude
- Model ID: claude-opus-4-6 (1M context)
- Capabilities: tool use, code editing

## 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 run tests locally and they pass
- [x] I have added or updated tests where applicable
- [ ] If this change affects the UI, I have included before/after
screenshots
- [x] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [x] I will address all Greptile and reviewer comments before
requesting merge

---------

Co-authored-by: Paperclip <noreply@paperclip.ing>

CONTRIBUTING.md

@@ -11,8 +11,9 @@ We really appreciate both small fixes and thoughtful larger changes.
 - Pick **one** clear thing to fix/improve
 - Touch the **smallest possible number of files**
 - Make sure the change is very targeted and easy to review
-- All automated checks pass (including Greptile comments)
-- No new lint/test failures
+- All tests pass and CI is green
+- Greptile score is 5/5 with all comments addressed
+- Use the [PR template](.github/PULL_REQUEST_TEMPLATE.md)
 
 These almost always get merged quickly when they're clean.
 
@@ -26,11 +27,26 @@ These almost always get merged quickly when they're clean.
   - Before / After screenshots (or short video if UI/behavior change)
   - Clear description of what & why
   - Proof it works (manual testing notes)
-  - All tests passing
-  - All Greptile + other PR comments addressed
+  - All tests passing and CI green
+  - Greptile score 5/5 with all comments addressed
+  - [PR template](.github/PULL_REQUEST_TEMPLATE.md) fully filled out
 
 PRs that follow this path are **much** more likely to be accepted, even when they're large.
 
+## PR Requirements (all PRs)
+
+### Use the PR Template
+
+Every pull request **must** follow the PR template at [`.github/PULL_REQUEST_TEMPLATE.md`](.github/PULL_REQUEST_TEMPLATE.md). If you create a PR via the GitHub API or other tooling that bypasses the template, copy its contents into your PR description manually. The template includes required sections: Thinking Path, What Changed, Verification, Risks, and a Checklist.
+
+### Tests Must Pass
+
+All tests must pass before a PR can be merged. Run them locally first and verify CI is green after pushing.
+
+### Greptile Review
+
+We use [Greptile](https://greptile.com) for automated code review. Your PR must achieve a **5/5 Greptile score** with **all Greptile comments addressed** before it can be merged. If Greptile leaves comments, fix or respond to each one and request a re-review.
+
 ## General Rules (both paths)
 
 - Write clear commit messages
@@ -41,7 +57,7 @@ PRs that follow this path are **much** more likely to be accepted, even when the
 
 ## Writing a Good PR message
 
-Please include a "thinking path" at the top of your PR message that explains from the top of the project down to what you fixed. E.g.:
+Your PR description must follow the [PR template](.github/PULL_REQUEST_TEMPLATE.md). All sections are required. The "thinking path" at the top explains from the top of the project down to what you fixed. E.g.:
 
 ### Thinking Path Example 1: