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-cli/src/cli/mod.rs

@@ -9,6 +9,7 @@ pub mod console;
 pub mod daemon;
 pub mod dialogs;
 pub mod doctor;
+pub mod emulate;
 pub mod ensure_daemon;
 pub mod error;
 pub mod evaluate;
@@ -36,6 +37,7 @@ use clap::{Args, Parser, Subcommand};
 
 use crate::cli::console::ConsoleArgs;
 use crate::cli::daemon::DaemonCmd;
+use crate::cli::emulate::EmulateArgs;
 use crate::cli::evaluate::EvaluateArgs;
 use crate::cli::get_html::GetHtmlArgs;
 use crate::cli::human_loop::RequestHelpArgs;
@@ -123,6 +125,9 @@ pub enum Command {
     /// Agent Window management commands.
     Window(WindowCmd),
 
+    /// Emulate a mobile device environment (viewport, UA, touch) on a tab.
+    Emulate(EmulateArgs),
+
     /// Capture a PNG of the active tab or a snapshot ref element.
     Screenshot(ScreenshotArgs),