docs: forbid internal Paperclip issue references in PRs (#8293)

## Thinking Path

> - Paperclip is the open source app people use to manage AI agents for
work
> - Contributions flow through GitHub PRs against
`paperclipai/paperclip`, but most contributors author their work inside
their own private Paperclip instance
> - Those instances mint instance-local issue ids and deep links
(`PAPA-123`, `/PAP/issues/...`, `agent://...`, localhost/tailnet URLs)
that only the author can open
> - When contributors paste those references into a public PR, reviewers
hit clutter and broken links and have to ask for cleanup
> - There was no written guidance telling contributors (human or agent)
to keep those references out
> - This pull request adds that guidance to `CONTRIBUTING.md` and
surfaces it as a PR-template checklist item plus an inline hint
> - The benefit is cleaner PRs that reference only public, openable
GitHub issues/PRs, with internal context restated in plain English

## Linked Issues or Issue Description

Fixes: #8292

## What Changed

- `CONTRIBUTING.md`: new **No Internal Issue References** subsection
explaining what to leave out (internal ticket ids, instance UI/deep
links, localhost/tailnet URLs) and to restate useful context in plain
English; added a cross-reference sentence from the existing "Link Issues
or Describe Them In-PR" guidance.
- `.github/PULL_REQUEST_TEMPLATE.md`: new checklist item, plus an inline
hint in the Linked Issues comment block, directing contributors to
reference only public GitHub issues/PRs.

## Verification

Docs-only change. Rendered Markdown locally and confirmed the new
`#no-internal-issue-references` anchor resolves from the cross-reference
link, and that the template checklist item appears in the rendered list.
No code paths affected.

## Risks

Low risk — documentation and PR-template text only; no runtime, build,
or schema impact. An optional automated CI gate was intentionally
deferred to a follow-up to avoid regex false positives.

## Model Used

Claude Opus 4.7 (claude-opus-4-7), extended thinking, agentic tool use
via Claude Code.

## 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] 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
- [x] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [x] All Paperclip CI gates are green
- [ ] 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

@@ -61,7 +61,21 @@ We do not gate PRs on a pre-existing issue. Two acceptable paths:
    - **Feature:** problem/motivation, proposed solution, alternatives considered, roadmap alignment. See [`feature_request.yml`](.github/ISSUE_TEMPLATE/feature_request.yml).
    - **New adapter:** agent or provider, why it's useful, how it's invoked. See [`adapter_request.yml`](.github/ISSUE_TEMPLATE/adapter_request.yml).
 
-Either way, a reviewer should be able to understand the underlying issue without leaving the PR. Commitperclip may check that one of these two paths is satisfied.
+Either way, a reviewer should be able to understand the underlying issue without leaving the PR. Commitperclip may check that one of these two paths is satisfied. Only link **public** GitHub issues — see [No Internal Issue References](#no-internal-issue-references) for what to leave out.
+
+### No Internal Issue References
+
+Many contributors run their own Paperclip instance to manage their work. Issue ids and links from *your* instance are private — reviewers and other contributors cannot open them, so they show up as clutter or broken links.
+
+In your PR title, description, commits, and comments, **only reference public GitHub issues and PRs** — `#123`, `Fixes #123` / `Closes #123` / `Refs #123`, or full `https://github.com/paperclipai/paperclip/...` URLs.
+
+Do **not** include references to internal/instance-local Paperclip work, such as:
+
+- Internal ticket ids like `PAPA-123`, `PAP-224`, or any `{PREFIX}-{NUMBER}` identifier that isn't a public GitHub issue number.
+- Instance UI links such as `/PAP/issues/...`, `/PAP/agents/...`, `agent://...`, or document deep links.
+- `localhost`, private IP, or tailnet URLs pointing at your own instance.
+
+If an internal issue captured useful context, restate that context in plain English in the PR body instead of linking to it.
 
 ### Model Used (Required)