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/commands/theme_cmd.rs

@@ -0,0 +1,41 @@
+//! theme: switch the active color theme and persist it for the next start.
+//!
+//! With no argument, prints the current theme and the available list.
+//! With a name, switches to that theme (also used by the REPL 'theme' command).
+
+use super::*;
+use crate::theme;
+
+pub fn run(app: &App, name: Option<&str>) -> Result<i32> {
+    if let Some(name) = name {
+        let theme = theme::find(name).ok_or_else(|| {
+            anyhow::anyhow!(
+                "unknown theme '{name}' — available: {}",
+                theme::names().join(", ")
+            )
+        })?;
+        app.set_theme(theme);
+        crate::commands::config_cmd::persist_setting(app, "settings.theme", theme.name)?;
+        app.log.success(&format!(
+            "theme set to {} ({})",
+            theme.name,
+            theme.title
+        ));
+        return Ok(0);
+    }
+
+    // No argument: show current and list.
+    let current = app.theme();
+    if app.json() {
+        crate::output::print_json(&serde_json::json!({
+            "current": current.name,
+            "title": current.title,
+            "available": theme::names(),
+        }));
+    } else {
+        println!("current theme: {} — {}", current.name, current.title);
+        println!("available themes: {}", theme::names().join(", "));
+        println!("use 'am theme <name>' to switch");
+    }
+    Ok(0)
+}