docs(rfc): define stable release policy

[drew]

Summary

Propose RFC 0014, defining OpenShell's alpha exit criteria and the stable release policy for per-commit development builds, nightly-generated release candidates, and weekly stable releases.

Related Issue

Supersedes #2148. RFC 0014 consolidates that earlier support-matrix and API-compatibility proposal into the broader release-stability design.

Changes

• Define the Tuesday stable cadence, per-commit development releases, nightly-generated release candidates, and artifact promotion rules.
• Establish compatibility, capability maturity, maintenance, and backport policies.
• Define the build matrix and blocking conformance, upgrade, breaking API change, and security qualification suites.
• Require canonical binary payloads across standalone downloads, installation packages, and container images.

Testing

• mise run pre-commit passes
• Unit tests added/updated (documentation-only change)
• E2E tests added/updated (not applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (not applicable; this PR proposes an RFC)