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

apps/extension/src/transport/types.ts

@@ -565,6 +565,63 @@ export interface RequestHelpResult {
   resolved_targets?: ResolvedTarget[];
 }
 
+// --------------------------------------------------------------------------
+// Device-emulation payloads — tool.emulate (mirror bsk-protocol emulate.rs)
+// --------------------------------------------------------------------------
+
+export interface UserAgentBrandVersion {
+  brand: string;
+  version: string;
+}
+
+/** Mirror of CDP `Emulation.UserAgentMetadata`; all fields optional. */
+export interface UserAgentMetadata {
+  brands?: UserAgentBrandVersion[];
+  full_version?: string;
+  platform?: string;
+  platform_version?: string;
+  architecture?: string;
+  model?: string;
+  mobile?: boolean;
+}
+
+/**
+ * Concrete emulation overrides for one tab. The extension merges each
+ * request field by field onto the tab's remembered emulation state:
+ * fields present here overwrite the stored value, absent fields keep
+ * it, and the merged state is applied as a whole.
+ */
+export interface EmulateOverrides {
+  width?: number;
+  height?: number;
+  device_scale_factor?: number;
+  mobile?: boolean;
+  user_agent?: string;
+  accept_language?: string;
+  user_agent_metadata?: UserAgentMetadata;
+  touch?: boolean;
+  max_touch_points?: number;
+}
+
+export interface EmulateParams {
+  session_id: string;
+  tab_id?: number;
+  /** Clear every emulation override on the tab. Exclusive with `overrides`. */
+  off?: boolean;
+  /** Overrides to apply. Required unless `off` is set. */
+  overrides?: EmulateOverrides;
+}
+
+export interface EmulateResult {
+  tab_id: number;
+  /** True when overrides were cleared (`off`); false when applied. */
+  cleared: boolean;
+  /** Echo of the overrides that were applied. Absent when cleared. */
+  applied?: EmulateOverrides;
+  /** Scope note: overrides are per-tab (CDP target), not inherited by new tabs. */
+  note?: string;
+}
+
 // --------------------------------------------------------------------------
 // Semantic record payloads — mirror bsk-protocol record.rs (Trace v2)
 // --------------------------------------------------------------------------