feat(tui): add onboarding tutorial with :tutorial command and first‑run UI

- Introduce `show_onboarding` UI setting (default true) and persist its state after first launch.
- Show onboarding status line and system status on initial run; fallback to normal status thereafter.
- Implement `show_tutorial` method displaying keybinding tips and system status.
- Register `:tutorial` command in command palette.
- Add migration documentation explaining `schema_version` update and deprecation of `agent.max_tool_calls`.
- Update README with description of the new tutorial command.

crates/owlen-core/src/config.rs

@@ -625,6 +625,8 @@ pub struct UiSettings {
     pub show_role_labels: bool,
     #[serde(default = "UiSettings::default_wrap_column")]
     pub wrap_column: u16,
+    #[serde(default = "UiSettings::default_show_onboarding")]
+    pub show_onboarding: bool,
 }
 
 impl UiSettings {
@@ -647,6 +649,10 @@ impl UiSettings {
     fn default_wrap_column() -> u16 {
         100
     }
+
+    const fn default_show_onboarding() -> bool {
+        true
+    }
 }
 
 impl Default for UiSettings {
@@ -657,6 +663,7 @@ impl Default for UiSettings {
             max_history_lines: Self::default_max_history_lines(),
             show_role_labels: Self::default_show_role_labels(),
             wrap_column: Self::default_wrap_column(),
+            show_onboarding: Self::default_show_onboarding(),
         }
     }
 }