Add self-update command

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "cassady"
-version = "0.2.6"
+version = "0.2.7"
 edition = "2021"
 description = "Cassady/Cass minimal terminal coding agent"
 license = "MIT"
@@ -33,11 +33,16 @@ ratatui = { version = "0.30", default-features = false, features = ["crossterm",
 regex = "1"
 pulldown-cmark = "0.12"
 reqwest = { version = "0.12", default-features = false, features = ["json", "rustls-tls", "stream"] }
+semver = "1"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
+sha2 = "0.10"
+flate2 = "1"
+tar = "0.4"
 thiserror = "1"
 tokio = { version = "1", features = ["macros", "rt-multi-thread", "sync", "time", "process", "io-util"] }
 unicode-width = "0.1"
+zip = { version = "2", default-features = false, features = ["deflate"] }
 
 [dev-dependencies]
 tempfile = "3"