chore(github): use the organization code of conduct

[ss-o]

Summary

• Remove the root code-of-conduct.md so the organization Code of Conduct from z-shell/.github applies. The removed file was a Contributor Covenant 1.4 copy from the upstream project and named the upstream author's address as its enforcement contact.
• Remove .prettierignore, whose only entry was that file.

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

Verification

• repo-settings-audit.rb --community-health with the pattern from fix(governance): match a hyphenated code-of-conduct file .github#738 lists this file for this repository as shadow with the blob that was removed.
• The file was compared with the organization Code of Conduct.
• No other file in the repository refers to the removed paths.

Agent handoff

• Status: Ready for review.
• Current state: Removal of a local code of conduct 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]

Review of head be66001.

Scope: deletion of code-of-conduct.md and .prettierignore.

• The removed file is the upstream Contributor Covenant 1.4 text. The organization Code of Conduct (Contributor Covenant 2.1) covers its pledge, standards, enforcement responsibilities, scope and reporting, with the organization contact in place of the upstream author's address. Two 1.4 sentences have no 2.1 counterpart: that maintainers who do not enforce the code in good faith may face repercussions, and that maintainers may further define representation of the project. Neither is specific to this repository.
• .prettierignore listed only the removed file, so deleting it changes no formatting scope for any remaining file.
• No other file in the repository refers to either path.
• The base is main of this repository at 5dad92a.

Checks on this head: 10 of 10 pass, including the required checks. No actionable findings.