feat(tty-local): initialize alknet-tty-local sibling crate (ADR-054)

Create crates/alknet-tty-local with Cargo.toml (alknet-tty workspace dep,
portable-pty, libc unix-only, tokio, bytes, futures-core, tracing,
thiserror), src/lib.rs with module declarations and LocalTtyBackend
re-export, and skeleton module files (pty, pipe, backend) with doc
comments and TODO markers. Add the crate to the workspace members list.

Cargo.toml

@@ -5,6 +5,7 @@ members = [
     "crates/alknet-call",
     "crates/alknet-http",
     "crates/alknet-tty",
+    "crates/alknet-tty-local",
 ]
 resolver = "2"