acp: IDE-proxied file system operations

[bug-ops]

Parent: #762

Task

Instead of direct FS access, proxy file reads/writes through the IDE client using ACP's fs/read_text_file and fs/write_text_file methods.

Acceptance criteria

• fs.rs — AcpFileSystem struct implementing IDE-proxied file I/O
• Send fs/read_text_file JSON-RPC request to client, await response
• Send fs/write_text_file JSON-RPC request to client, await response
• Handle absolute path requirement (all paths MUST be absolute per spec)
• Diff generation for file edits (old content vs new content)
• Error handling for client-side failures (file not found, permission denied)
• Adapter to make AcpFileSystem usable by FileExecutor or as a standalone tool