feat(tty-local): implement LocalTtyBackend (TtyBackend) branching on terminal Some/None

Implements the wiring task: LocalTtyBackend::allocate() dispatches to
pty::allocate_pty when TtyParams.terminal is Some (PTY mode, stderr None)
and pipe::allocate_pipe when None (runner mode, stderr Some). Validates
non-empty cmd, ignores backend_params (local backend has no backend-
specific selector fields), and resource_id() returns None (local backend
creates its own resource — process). Adds async-trait dependency.

crates/alknet-tty-local/Cargo.toml

@@ -11,6 +11,7 @@ name = "alknet_tty_local"
 
 [dependencies]
 alknet-tty = { path = "../alknet-tty" }
+async-trait = "0.1"
 portable-pty = "0.9"
 tokio = { version = "1", features = ["full"] }
 tokio-stream = "0.1"