docs: update CONTRIBUTING.md to require PR template, Greptile 5/5, and passing tests

Add explicit PR Requirements section referencing .github/PULL_REQUEST_TEMPLATE.md.
Clarify that all PRs must use the template, achieve a 5/5 Greptile score with
all comments addressed, and have passing tests/CI before 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). The most important part is the "thinking path" at the top — it explains from the top of the project down to what you fixed. E.g.:
 
 ### Thinking Path Example 1: