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

tests/doctor_test.rs

@@ -11,7 +11,7 @@ fn report_contains_core_checks() {
     let runner = SystemRunner::new(false, false, &app.log);
     let report = doctor::run(&app, &runner);
     let names: Vec<&str> = report.checks.iter().map(|c| c.name.as_str()).collect();
-    for expected in ["git", "config", "config-schema", "state", "install-dir", "bin-on-path"] {
+    for expected in ["git", "config", "config-schema", "state", "install-dir", "bin-on-path", "ollama", "llama-server"] {
         assert!(
             names.contains(&expected),
             "missing check '{expected}' in {names:?}"