feat: complete protocol spec and initial implementation

- Write PROTOCOL.md with full wire format spec and 8 real-world scenario
  analyses (reconnect, multi-operator, large files, AV evasion, router crash,
  malformed packets, future pivoting)

- Rewrite workspace structure:
  - unshell lib: protocol types (PacketHeader, TreeRequest/Response,
    HandshakeMessage/Ack), Transport trait, TcpTransport, Tree routing
  - ush-router: router binary with per-node threads, NodeRegistry with
    longest-prefix path matching, packet relay
  - ush-payload: implant binary with reconnect loop, module tree, InfoModule
  - ush-cli: operator REPL with rustyline, session management, command parser

- Protocol design: two-part rkyv frame [header][payload]; router reads only
  header for routing, payload bytes forwarded opaque

- All code documented with doc comments and examples
- Zero warnings, zero errors across entire workspace
- 32 tests pass (unit tests for tree routing, TCP transport, framing,
  command parsing, node registry)

src/protocol/mod.rs

@@ -0,0 +1,40 @@
+//! # Protocol Module
+//!
+//! All wire types used by the UnShell protocol.
+//!
+//! ## Module layout
+//!
+//! ```text
+//! protocol/
+//!   mod.rs      ← you are here; re-exports everything
+//!   types.rs    ← PacketHeader, TreeRequest, TreeResponse, Handshake*
+//!   content.rs  ← content-type string constants
+//! ```
+//!
+//! ## Quick start
+//!
+//! ```rust
+//! use unshell::protocol::{
+//!     PacketHeader, PacketType,
+//!     TreeRequest, RequestType,
+//!     content,
+//! };
+//!
+//! let header = PacketHeader {
+//!     dst_path: "/agents/abc123/shell/exec".into(),
+//!     src_path: "/operator/sess1".into(),
+//!     packet_type: PacketType::Request,
+//! };
+//!
+//! let request = TreeRequest {
+//!     request_id: 1,
+//!     request_type: RequestType::CallProcedure,
+//!     content_type: content::UTF8_STRING.into(),
+//!     data: b"ls -la".to_vec(),
+//! };
+//! ```
+
+pub mod content;
+mod types;
+
+pub use types::*;