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/config.rs

@@ -102,6 +102,35 @@ pub struct Settings {
     /// lines (global; project profiles can add their own).
     #[serde(default)]
     pub hooks: BTreeMap<String, Vec<String>>,
+    /// Update policy: how aggressively 'am update' should act (issue #65).
+    #[serde(default)]
+    pub update_policy: Option<UpdatePolicy>,
+}
+
+/// Policy controlling how aggressively 'am update' upgrades installed agents.
+#[derive(Debug, Clone, Copy, Serialize, Deserialize, Default, PartialEq, Eq)]
+#[serde(rename_all = "lowercase")]
+pub enum UpdatePolicy {
+    /// Never update automatically; 'am update --all' is a no-op.
+    None,
+    /// Default: update only when explicitly requested by name or with --all.
+    #[default]
+    Manual,
+    /// Update only to the next patch/minor that is known safe (future use).
+    Safe,
+    /// Always update to the latest available version.
+    Latest,
+}
+
+impl UpdatePolicy {
+    pub fn as_str(&self) -> &'static str {
+        match self {
+            UpdatePolicy::None => "none",
+            UpdatePolicy::Manual => "manual",
+            UpdatePolicy::Safe => "safe",
+            UpdatePolicy::Latest => "latest",
+        }
+    }
 }
 
 /// A project profile: everything am needs to know about one project.
@@ -172,6 +201,9 @@ pub struct AgentDef {
     /// Known version (otherwise detected via --version).
     #[serde(default)]
     pub version: Option<String>,
+    /// Pinned version: 'am update' will refuse to install a different one (issue #65).
+    #[serde(default)]
+    pub pin_version: Option<String>,
     #[serde(default)]
     pub tags: Vec<String>,
     /// When false, the agent is listed but cannot be installed locally.