docs: add project governance

[johntmyers]

Summary

Add project governance for OpenShell based on the CNCF maintainer governance template and informed by Cilium's contributor ladder. The document defines project values, contributor roles, maintainer eligibility and responsibilities, communication, voting, inactivity, and governance evolution.

Related Issue

No issue required: this PR adds the governance documentation needed to formalize the project's existing maintainer model and CNCF preparation.

Changes

• Define the Contributor, Reviewer, and Maintainer ladder
• Establish minimum Maintainer eligibility while making clear that raw activity metrics do not confer Maintainer status
• Require broad platform stewardship, sound technical judgment, community-first conduct, and accountability for AI-assisted work
• Document nomination, voting, removal, and six-month inactivity processes
• Link community meeting information, the Maintainer mailing list, lazy consensus guidance, the CNCF Code of Conduct, and the security policy

Testing

• mise run markdown:lint passes
• git diff --check passes
• mise run pre-commit passes (not run to completion for this Markdown-only change, at requester direction)
• Unit tests added/updated (not applicable; documentation only)
• E2E tests added/updated (not applicable; documentation only)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (not applicable; no architecture change)