arch: split ownership of content-panel definitions

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Problem

Panel definitions are split across two homes, making ownership unclear and encouraging features to depend directly on shell internals.

• Files panels live in features/files/ and import shell APIs from components/layout/content-panel/
• Peer panels (terminal, diff, browser) live under components/layout/content-panel/panels/
• Registration happens in the composition root (routes/__root.tsx)

Severity

P3

Suggested fix

Pick one convention:

• Move filesPanel / filePanel under components/layout/content-panel/panels/ and keep only view logic in features/files/, or
• Move all panel definitions into their owning features and keep components/layout/content-panel/ as host infrastructure only.

Features should not need to reach into shell panel wiring beyond what the composition root imports.