docs: add PR rules for file size and code comments (#16036)

## Thinking Path

> - Paperclip is the open source app people use to manage AI agents for
work.
> - Contributors use CONTRIBUTING.md to prepare pull requests.
> - The guide is missing two code submission rules.
> - Contributors need to see these rules before they submit changes.
> - This pull request adds the file size and code comment rules to the
requirements for all PRs.
> - The benefit is a clear statement of the requested contribution
policy.

## Linked Issues or Issue Description

**Issue type**

Missing documentation.

**Where is the issue?**

CONTRIBUTING.md, PR Requirements (all PRs).

**What's wrong?**

The guide does not state the requested limits on file growth and code
comments.

**Suggested fix**

Add both rules with the wording requested by the maintainer. No related
issue or PR was found in the GitHub search.

## What Changed

- Add: "Do not submit a PR that moves a code file from less than 1,000
lines of code to more than 1,000 lines of code."
- Add: "Do not submit a PR that includes code comments."

## Verification

- Confirm both rules appear under PR Requirements (all PRs) in
CONTRIBUTING.md.
- `git diff --check origin/master...HEAD` passed.
- Greptile passed on commit `8cc5cd4be63cfbb16b3265b39e487581387bc3d0`
with 5/5 and no actionable findings.
- Local `pnpm -r typecheck`, `pnpm test:run`, and `pnpm build` were
attempted. This worktree has no installed dependencies, so typecheck
cannot resolve Node types and tests/build cannot load tsx. GitHub CI
results will be checked before merge.

## Risks

- Low risk. This change adds documentation only.
- The wording states contribution policy without adding an automated
gate.

## Model Used

- OpenAI GPT-6 via Codex. Used reasoning, shell tools, and file editing.
The exact runtime model ID and context window are not exposed in this
session.

## 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 either (a) linked existing issues with `Fixes: #` / `Closes
#` / `Refs #` OR (b) described the issue in-PR following the relevant
issue template
- [x] I have not referenced internal/instance-local Paperclip issues or
links (only public GitHub `#NNN` / `github.com/paperclipai/paperclip`
URLs)
- [x] My branch name describes the change (e.g. `docs/...`, `fix/...`)
and contains no internal Paperclip ticket id or instance-derived details
- [x] I have run tests locally and they pass (not applicable to this
documentation-only change; the full commands were attempted but blocked
by missing local dependencies)
- [x] I have added or updated tests where applicable
- [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
- [x] Greptile is 5/5 with no open P2s, recommendations, or follow-ups
- [x] I will address all Greptile and reviewer comments before
requesting merge

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

CONTRIBUTING.md

@@ -46,6 +46,9 @@ PRs that follow this path are **much** more likely to be accepted, even when the
 
 ## PR Requirements (all PRs)
 
+- Do not submit a PR that moves a code file from less than 1,000 lines of code to more than 1,000 lines of code.
+- Do not submit a PR that includes code comments.
+
 ### 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, Model Used, and a Checklist.