feat(update): add check-only release diagnostics

Add `codewhale update --check` so users can compare the installed version with
  the latest release without downloading or replacing binaries.

  Surface the same release check in `codewhale doctor`, and share release lookup,
  mirror handling, timeout, and version comparison logic between update and doctor.

Cargo.toml

@@ -9,6 +9,7 @@ members = [
     "crates/hooks",
     "crates/mcp",
     "crates/protocol",
+    "crates/release",
     "crates/secrets",
     "crates/state",
     "crates/tools",