Surface remote source-control version drift

[jln13x]

Summary

• show a persistent version-mismatch indicator in the worktree source-control view
• distinguish actual client/server drift from missing source-control capabilities
• keep the compatibility fallback read-only while explaining how to restore mutation actions
• document the fork-specific behavior

Verification

• vp check
• vp run typecheck
• vp test apps/web/src/components/WorktreeSourceControl.logic.test.ts apps/web/src/versionSkew.test.ts