feat(app-server): add exclusive filesystem copies

[nornagon-openai]

Why

Codex Apps adapts app-server filesystem operations to its execution-host interface. Callers need an atomic copy mode that refuses to replace an existing destination and reports that condition consistently across local and remote hosts.

What

• Add exclusive to fs/copy, defaulting to false.
• Implement exclusive regular-file copies with create-new destination semantics so an existing file is never overwritten.
• Preserve AlreadyExists across exec-server RPC and expose structured app-server error data as { "code": "EEXIST" }.
• Cover local, remote, sandbox transport, and app-server destination-preservation behavior.
• Update generated protocol schemas and API documentation.

This PR is intentionally limited to exclusive file copies. File-size metadata and symlink semantics are independent changes.

Validation

• just write-app-server-schema
• just test -p codex-app-server-protocol
• just test -p codex-exec-server file_system_copy_exclusive_preserves_existing_destination
• just test -p codex-app-server fs_copy_exclusive_preserves_existing_destination
• just fix -p codex-file-system -p codex-exec-server -p codex-app-server-protocol -p codex-app-server
• just fmt