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

skill/SKILL.md

@@ -134,6 +134,22 @@ Details and flags: **`bsk <cmd> --help`**
 |---------|---------|
 | `bsk window resize` | Resize the Agent Window (`--width`, `--height`; 100..=7680 CSS px) |
 
+### Device emulation — `bsk emulate` (requires `--session <id>`)
+
+Emulate a mobile device environment on the agent tab via CDP — viewport, User-Agent, and touch — to debug a page's mobile layout and behaviour:
+
+```bash
+bsk emulate --session <id> --device iphone-14
+bsk emulate --session <id> --width 390 --height 844 --dpr 3 --mobile --ua "Mozilla/5.0 (iPhone…" --touch
+bsk emulate --session <id> --off
+```
+
+- Presets (`--device`): `iphone-14`, `iphone-14-pro-max`, `iphone-se`, `pixel-7`, `galaxy-s23`, `ipad-mini`, `galaxy-tab-s8`. Manual flags (`--width`/`--height`/`--dpr`/`--mobile`/`--ua`/`--accept-language`/`--touch`/`--max-touch-points`) also work without a preset, or override individual preset fields; `--no-mobile`/`--no-touch` turn a preset's mobile viewport / touch emulation back off.
+- Repeated runs merge field by field onto the tab's current emulation state — only the flags you pass change. E.g. after `--device iphone-14`, `bsk emulate --session <id> --width 390 --height 844` keeps the preset's dpr (`3`) and mobile viewport. (The extension remembers the state per tab; after an extension reload the next run applies only the fields it carries.)
+- `--off` clears every override (viewport, UA, touch) and the remembered state, restoring the tab's real environment.
+- Scope: overrides apply to **one tab only** (CDP per-target) and are **not inherited by new tabs** — re-run `bsk emulate` after opening or switching to another tab (default target is the session's active tab; `--tab-id` overrides).
+- Emulation covers viewport/UA/touch only; it does not throttle the network, fake geolocation, or synthesise real touch-event streams.
+
 ### Tabs (require `--session <id>`)
 
 | Command | Summary |