improve the agent content protocol

[bsutton]

High: The agent content-key protocol copies keys through normal heap strings/vectors. ContentKeyStore exposes Vec, encode_put() hex-encodes keys into
String, parse_request() decodes into Vec, and GET responses format keys as "KEY ". See rust/lockbox_vault/src/lib.rs:76, rust/lockbox_vault/src/
lib.rs:286, rust/lockbox_vault/src/lib.rs:347, rust/lockbox_vault/src/lib.rs:359, rust/lockbox_vault/src/unix.rs:136, and rust/lockbox_vault/src/
windows.rs:316. The cache stores SecretBytes, but the IPC path defeats that.

Medium: Unix agent transport security is effectively unchecked. verify_agent_transport_security() always returns Ok(()), and handle_client() does not
verify peer uid/gid. The implementation relies on socket directory permissions, but the directory may be env-selected via LOCKBOX_AGENT_DIR, chmod errors
are ignored, and there is no ownership/mode validation. See rust/lockbox_vault/src/unix.rs:56, rust/lockbox_vault/src/unix.rs:125, and rust/
lockbox_vault/src/unix.rs:191. Add peer credential checks and make the doctor check real.