chore: import upstream snapshot with attribution

CONTRIBUTING.md

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+# Contributing
+
+Thank you for your interest in contributing to Perspective!
+
+## Guidelines
+
+When submitting or commenting on an Issue, please respect the following
+guidelines. Github Issues are Perspective's project record of bugs and feature
+development, e.g. for publishing a release's Changelog, and as such it is
+important to keep them informative and on-topic. As such, please understand that
+we may remove or reclassify comments, Issues or PRs which violate the
+guidelines.
+
+Please note that due to the we may close your Issue or Pull Request for one of
+the reasons listed here or in the associated contribution template. If you find
+your contribution closed with a link to this document or a contribution
+template, please make sure you've followed the instructions closely before
+re-submitting.
+
+- Be respectful and civil!
+- Use the provided Issue and Pull Request templates. If the templates don't fit
+  your need, please open a
+  [discussion](https://github.com/perspective-dev/perspective/discussions)
+  instead.
+- Don't ask for issues to be assigned to you if you're a first-time contributor.
+  If you need help picking an issue to work on, please open a
+  [discussion](https://github.com/perspective-dev/perspective/discussions).
+- Don't add comments asking when a feature will be delivered or a reported issue
+  fixed. The Issue will link any in-progress draft PRs or Milestones (if known).
+
+When submitting a Pull Request (PR), please respect the following coding
+guidelines:
+
+- Don't open a PR without an associated
+  [Open Issue](https://github.com/perspective-dev/perspective/issues) which has
+  been tagged by a project maintainer.
+- Make sure your PR passes _build_, _test_ and _lint_ steps _completely_ before
+  opening a PR. Make _sure_ you've run these locally, even if you think your
+  change will not impact this step!
+- Sign commits (e.g. with `-s`) in accordance with the DCO policy detailed
+  below, _before_ opening a PR.
+- Please make sure PRs include the following _not optional_ components:
+    - Tests asserting behavior of any new or modified features.
+    - Docs for any new or modified public APIs.
+    - [Benchmarks](https://perspective-dev.github.io/docs/development/#benchmark)
+      for any performance-critical changes.
+
+- Keep PRs clean, simple and to-the-point:
+    - Squash "WIP", "Reverting ..", etc., commits.
+    - No merge commits (`git merge master`), prefer `rebase` to resolve
+      conflicts with the `master` branch.
+    - Try to organize commits as functional components (as opposed to
+      timeline-of-development).
+
+## DCO
+
+> [!IMPORTANT]
+>
+> Signed commits are required for all PRs.
+
+The Perspective project requires contributors to affirm their contributions via
+a [Developer Certificate of Origin](https://developercertificate.org), which
+certifies that developers are authorized to make their contribution, either on
+their own behalf or on behalf of their employer.
+
+In practice, this means that all commits to Perspective must be signed (e.g.
+with `-s`/`-S`). Pull requests with any unsigned commits, or where the signer
+does not match the commit author, can not be merged by Perspective's committers.
+We ask that you check that you have a signed all your commits before making a
+pull request. A [DCO enforcement bot](https://github.com/apps/dco) will
+automatically scan and flag any pull requests that lack a valid sign-off.
+
+If you have any general questions about contributing to Perspective, please feel
+free to open a discussion on
+[github](https://github.com/perspective-dev/perspective/discussions)
+
+## AI Assistance Notice
+
+This section was forked from
+[`ghostty`](https://github.com/ghostty-org/ghostty),
+[MIT License](https://github.com/ghostty-org/ghostty/blob/main/LICENSE)
+([File](https://github.com/ghostty-org/ghostty/pull/8289/files))
+([PR](https://github.com/ghostty-org/ghostty/pull/8289)). Thanks @mitchellh!
+
+> [!IMPORTANT]
+>
+> If you are using **any kind of AI assistance** to contribute to Perspective,
+> it must be disclosed in the pull request.
+
+If you are using any kind of AI assistance while contributing to Perspective,
+**this must be disclosed in the pull request**, along with the extent to which
+AI assistance was used (e.g. docs only vs. code generation). If PR responses are
+being generated by an AI, disclose that as well. As a small exception, trivial
+tab-completion doesn't need to be disclosed, so long as it is limited to single
+keywords or short phrases.
+
+An example disclosure:
+
+> This PR was written primarily by Claude Code.
+
+Or a more detailed disclosure:
+
+> I consulted ChatGPT to understand the codebase but the solution was fully
+> authored manually by myself.
+
+Failure to disclose this is first and foremost rude to the human operators on
+the other end of the pull request, but it also makes it difficult to determine
+how much scrutiny to apply to the contribution.
+
+In a perfect world, AI assistance would produce equal or higher quality work
+than any human. That isn't the world we live in today, and in most cases it's
+generating slop. I say this despite being a fan of and using them successfully
+myself (with heavy supervision)!
+
+Please be respectful to maintainers and disclose AI assistance.
+
+## Final Pull Request Checklist
+
+Before opening a Pull Request, be sure:
+
+- Includes a thorough Description which clearly states what problems the PR
+  solves.
+- Description contains a link to the Github Issue, and any relevent Discussions,
+  this PR applies to.
+- Include new tests that fail without this PR but passes with it.
+- Include any relevent Documentation changes related to this change.
+- Verify all commits have been _signed_ in accordance with the DCO policy.
+- Disclosed AI tooling assistance.
+- Reviewed PR commit history to remove unnecessary changes.
+- Make sure your PR passes _build_, _test_ and _lint_ steps _completely_.