[Winterrks]

Docs only, no code changes.

• docs/claude/release.md: release.ts requires COWORK_NATIVE=omit or both COWORK_LIBKRUN_INCLUDE and COWORK_LIBKRUN_ARCHIVE; what each mode produces and which combinations refuse before any build. Rollback section names schema v21.
• docs/claude/settings-and-migrations.md and docs/RELEASE-NOTES-cowork-schema.md (renamed from the v18 file to match main's name): schema v21, what v18 to v21 add, rollback accepts v8..v21, intermediate stores need the same step. Strings follow main's schema-docs-pin test.
• docs/claude/cowork.md and docs/cowork-status.md: libkrun SH2, host-backed shares (SH6) and /workspace/folders/, plan view, environment /etc/gitconfig, memory root, cold resume (host port still gated off), reproducible pinned guest image and runtimeImageDigest; export writer and restore exist behind ports. Not done: session.create cowork still unsupported_topology, egress broker and provider bridge held (owner's), gates G0-G9 not run.
• CLAUDE.md: two index rows and the Cowork line.