fix(extensions): isolate legacy sidebar files

[Tethys0]

Summary

• adapt deprecated sidebar views to receive a mutable copy of host-owned files
• preserve the readonly projection for modern panes
• add a render-level regression test for legacy files.sort()

Closes #764

Verification

• npx --yes bun test src/ui/AppHost.extension-sidebar.test.tsx --test-name-pattern "legacy sidebar can sort" (RED before fix: frozen-array TypeError; GREEN after)
• npx --yes bun test src/ui/AppHost.extension-sidebar.test.tsx
• npx --yes bun run format:check
• git diff --check

Scope

This preserves the deprecated mutable ExtensionSidebarViewProps.files contract without changing modern ExtensionPaneProps.files.

[greptile-apps]

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