feat(emulate): add mobile device emulation via CDP Emulation domain (#60)

* feat(emulate): add mobile device emulation via CDP Emulation domain

Add `bsk emulate` so agents can debug mobile page behaviour by
emulating a device environment on a session tab:

- Viewport metrics (Emulation.setDeviceMetricsOverride), User-Agent
  (Emulation.setUserAgentOverride, optional accept-language and UA
  client-hints metadata), and touch (Emulation.setTouchEmulationEnabled
  with maxTouchPoints).
- Seven CLI-side device presets (iphone-14, iphone-14-pro-max,
  iphone-se, pixel-7, galaxy-s23, ipad-mini, galaxy-tab-s8) via
  --device; manual flags (--width/--height/--dpr/--mobile/--ua/
  --accept-language/--touch/--max-touch-points) work standalone or
  override individual preset fields.
- `bsk emulate --off` clears every override (clearDeviceMetricsOverride
  + touch disabled + empty UA override) and restores the real
  environment.
- Overrides are per-tab (CDP per-target) and not inherited by new tabs;
  the result JSON carries a note reminding agents to re-apply.

Wire: new tool.emulate method end-to-end (bsk-protocol types + JSON
schemas + Method::ToolEmulate classified BrowserMutation, daemon
forwarding, CLI subcommand, extension tool handler + ChromiumCdp
Emulation wrappers, dispatcher registration), docs in both SKILL.md
copies, and unit tests on every layer.

* refine(emulate): merge per-tab state and align preset override semantics

- extension: remember the applied emulation state per tab and merge each new request onto it field by field before applying, so a later --width/--height no longer resets an earlier preset's dpr/mobile; --off clears the stored state along with the CDP overrides
- extension: cdp_failed errors now note that already-applied overrides were not rolled back and can be reset with --off
- cli: validate width/height pairing and metric requirements on the merged preset+flags result, so --device iphone-14 --width 500 keeps the preset height
- cli: add --no-mobile/--no-touch (conflicting with --mobile/--touch) to turn preset mobile/touch fields back off
- docs: describe the merge behaviour, --no-mobile/--no-touch and --off state clearing in both SKILL.md copies

crates/bsk-protocol/src/method.rs

@@ -46,6 +46,8 @@ pub enum Method {
     ToolSessionStop,
     #[serde(rename = "tool.window_resize")]
     ToolWindowResize,
+    #[serde(rename = "tool.emulate")]
+    ToolEmulate,
     #[serde(rename = "tool.tab_list")]
     ToolTabList,
     #[serde(rename = "tool.tab_create")]
@@ -143,6 +145,7 @@ impl Method {
             | Method::ToolTabReturn
             | Method::ToolTabSelect
             | Method::ToolWindowResize
+            | Method::ToolEmulate
             | Method::ToolNavigate
             | Method::ToolNavigateBack
             | Method::ToolNavigateForward
@@ -234,6 +237,13 @@ mod tests {
         assert_eq!(serde_json::to_value(method).unwrap(), json!("tool.network"));
     }
 
+    #[test]
+    fn emulate_method_round_trips() {
+        let method: Method = serde_json::from_value(json!("tool.emulate")).unwrap();
+        assert_eq!(method, Method::ToolEmulate);
+        assert_eq!(serde_json::to_value(method).unwrap(), json!("tool.emulate"));
+    }
+
     #[test]
     fn cancel_params_and_result_round_trip() {
         let params: CancelParams = serde_json::from_value(json!({ "rpc_id": "wait-1" })).unwrap();
@@ -276,6 +286,7 @@ mod tests {
         assert!(Method::ToolEvaluate.is_mutating());
         assert!(Method::ToolRecordStart.is_mutating());
         assert!(Method::ToolWindowResize.is_mutating());
+        assert!(Method::ToolEmulate.is_mutating());
     }
 
     #[test]