docs: require descriptive, instance-free PR branch names (#8312)

## 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
> - That tooling names a working branch after the internal issue/task —
e.g. `PAPA-42-why-did-this-break`
> - When that branch is pushed for a PR, the public PR's head branch
carries instance-local context: it's not meaningful to reviewers and
leaks instance-derived identifiers
> - We just documented the issue-reference side of this (#8292), but
said nothing about branch names
> - This pull request adds a **Branch Naming** section to
`CONTRIBUTING.md` and a matching PR-template checklist item, telling
contributors to rename branches to descriptive, change-scoped names
before pushing
> - The benefit is public PRs whose branch names describe the change and
contain no instance-local details

## Linked Issues or Issue Description

Fixes: #8311
Refs: #8292

## What Changed

- `CONTRIBUTING.md`: new **Branch Naming** subsection — rename
instance-named branches (e.g. `PAPA-42-...`) to descriptive,
change-scoped, kebab-case names before pushing, with examples and the
rename commands.
- `.github/PULL_REQUEST_TEMPLATE.md`: new checklist item asserting the
branch name describes the change and contains no internal ticket id or
instance-derived details.

## Verification

Docs-only change. Rendered Markdown locally and confirmed the new
**Branch Naming** section and the new checklist item appear as intended.
This PR's own branch (`docs/contributor-branch-naming`) follows the new
guidance. No code paths affected.

## Risks

Low risk — documentation and PR-template text only; no runtime, build,
or schema impact. Automatically renaming branches at push time in the
tooling is intentionally out of scope and tracked as separate follow-up
work.

## 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] My branch name describes the change and contains no internal
Paperclip ticket id or instance-derived details
- [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
- [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>

.github/PULL_REQUEST_TEMPLATE.md

@@ -88,6 +88,7 @@
 - [ ] 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 not referenced internal/instance-local Paperclip issues or links (only public GitHub `#NNN` / `github.com/paperclipai/paperclip` URLs)
+- [ ] My branch name describes the change (e.g. `docs/...`, `fix/...`) and contains no internal Paperclip ticket id or instance-derived details
 - [ ] 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

CONTRIBUTING.md

@@ -77,6 +77,24 @@ Do **not** include references to internal/instance-local Paperclip work, such as
 
 If an internal issue captured useful context, restate that context in plain English in the PR body instead of linking to it.
 
+### Branch Naming
+
+Tooling (including Paperclip) often names a working branch after an internal issue and task — e.g. `PAPA-42-why-did-this-break`. That name leaks instance-local context, isn't meaningful to reviewers, and ends up as the public branch on your PR.
+
+Before you push, **rename the branch to something descriptive of the change itself**, not of your instance:
+
+- Use short, kebab-case names scoped to the change, optionally with a conventional prefix: `docs/no-internal-issue-references`, `fix/sandbox-secret-resolution`, `feat/adapter-retry-backoff`.
+- Do **not** include internal Paperclip ticket ids (`PAPA-123`, `PAP-224`), instance task slugs, or other instance-derived details in the branch name.
+
+To rename and push under the new name:
+
+```bash
+git branch -m <descriptive-name>
+git push -u origin <descriptive-name>
+# If your tooling already pushed the old branch, delete it from origin:
+git push origin --delete <old-name>
+```
+
 ### Model Used (Required)
 
 Every PR must include a **Model Used** section specifying which AI model produced or assisted with the change. Include the provider, exact model ID/version, context window size, and any relevant capability details (e.g., reasoning mode, tool use). If no AI was used, write "None — human-authored". This applies to all contributors — human and AI alike.