chore(github): use the organization contributing guide

[ss-o]

Summary

• Remove the root contributing.md so the organization contributing guide from z-shell/.github applies. The removed guide was written for the upstream project: it linked labels and a chat room of that repository and described a next and master branch model this repository does not use.
• Drop its entry from .prettierignore.

Refs z-shell/.github#717 and z-shell/.github#720.

Verification

• repo-settings-audit.rb --community-health from z-shell/.github listed this file for this repository with the blob that was removed.
• The file was compared with the organization contributing guide.
• No other file in the repository refers to the removed path.

Agent handoff

• Status: Ready for review.
• Current state: Removal of a local contributing guide on a signed head.
• Blockers: A review of record on the current head.
• Next steps: Merge.
• Related tracker/issues: refactor(agents): organize knowledge and routing by domain .github#717, feat(meta): standardize org-wide templates, repository envelope, and fallback review prompting .github#720.

[ss-o]

Fallback review under ADR-0026: Copilot request not registered on 784d040

Findings

No actionable findings.

Scope: the complete diff of 784d040e against base main at 924081c1 (merge base), 2 files, +0/-66, 2 commits. Specification: z-shell/.github#720.

Note

This is a maintainer-elected fallback self-review after the Copilot request did not register. It is not an independent review or approval to merge.

Review request record

• 2026-10-06T10:57:15Z: POST /repos/z-shell/zunit/pulls/38/requested_reviewers with copilot-pull-request-reviewer[bot] on head 784d040 returned success with an empty requested_reviewers list.
• No Copilot review request or review exists on the pull request. This pull request is not a draft. Review request not registered; cause unconfirmed.

Validation

• Hosted: 10 of 10 checks pass on this head, including Org Routing, Trunk Code Quality, zunit and Zsh Lint.
• Passed, local on this head: the removed blob equals the one the organization community health audit listed; the file was compared with the organization contributing guide; no tracked file refers to the removed path.
• Signature: both commits are signed and GitHub reports them verified.

Checklist verdicts and supporting evidence

• contributing.md: pass. Written for the upstream project (its labels, chat room and a next and master branch model this repository does not use); the organization guide applies instead.
• .prettierignore: pass. Only the entry for the removed file goes.
• Security: not applicable. Documentation and tool configuration only.

Limits and follow-up

• Any push to this branch voids this review.