feat: phase 2 quick wins (#65 #69 #72)

- #65: update policies + per-agent version pinning
  - add settings.update_policy (stable/latest/skip)
  - add AgentDef.pin_version
  - honor both in update_cmd
  - add update_policy tests

- #69: REPL color theme CLI command
  - add am theme [name] (show/switch)
  - persist selected theme to config

- #72: doctor checks local model runtimes
  - verify ollama and llama-server on PATH
  - for ollama, also check server responds (ollama ls)

Version 0.4.4

src/cli.rs

@@ -157,6 +157,11 @@ pub enum Command {
         #[arg(long, value_name = "DIR")]
         output: Option<PathBuf>,
     },
+    /// Set or show the active color theme (persisted for the next run)
+    Theme {
+        /// Theme name (omit to list available themes)
+        name: Option<String>,
+    },
     /// Internal: list agent names for shell completions (hidden)
     #[command(name = "_agents", hide = true)]
     Agents {