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

[BB-fat]

Motivation

Agents driving real user browsers had no way to debug mobile page behaviour: the codebase made no use of the CDP Emulation domain at all — no viewport metrics override, no UA spoofing, no touch emulation. (Window sizing from #52 only resizes the real window; pages still see a desktop environment.)

What changes

New bsk emulate command + tool.emulate end-to-end:

bsk emulate --session <id> --device iphone-14        # built-in preset
bsk emulate --session <id> --width 390 --height 844 --dpr 3 --mobile --ua "Mozilla/5.0 (iPhone…)"
bsk emulate --session <id> --device pixel-7 --touch  # manual flags layer on top of presets
bsk emulate --session <id> --off                     # clear every override

• Viewport via Emulation.setDeviceMetricsOverride (width/height/deviceScaleFactor/mobile) — changes what the page sees (innerWidth, media queries, meta-viewport handling), unlike window resizing.
• User-Agent via Emulation.setUserAgentOverride, with optional accept-language and UA client-hints metadata (protocol-level).
• Touch via Emulation.setTouchEmulationEnabled + maxTouchPoints, so ontouchstart / pointer media queries behave like a touch device.
• Seven CLI-side presets: iphone-14, iphone-14-pro-max, iphone-se, pixel-7, galaxy-s23, ipad-mini, galaxy-tab-s8. Manual flags work standalone or override individual preset fields.
• Overrides are per-tab (CDP per-target) and not inherited by new tabs; the result JSON carries a note so agents know to re-apply.

Explicitly out of scope (possible v2): real touch event streams (Input.dispatchTouchEvent), network throttling, geolocation.

Implementation

• bsk-protocol: tool.emulate method (classified BrowserMutation), EmulateParams/EmulateOverrides/EmulateResult types + regenerated JSON schemas.
• CLI: new emulate subcommand (presets, validation, JSON/human output), daemon forwards ToolEmulate to the extension.
• Extension: ChromiumCdp Emulation wrappers, new tools/emulate.ts handler (off clears; otherwise viewport → UA → touch applied in order), dispatcher registration, transport types mirrored.
• Docs: both SKILL.md copies gain a concise "Device emulation" section (presets, --off, per-tab scope).

Tests

• New: 6 protocol serde tests, 15 CLI param-building unit tests, 4 cli_parse integration tests, 22 extension vitest cases for the handler.
• cargo fmt --all --check, clippy -p bsk-protocol -p bsk --all-targets -D warnings: clean.
• cargo test -p bsk-protocol: 113 passed. cargo test -p bsk --lib: 207 passed; sync_continues_on_partial_error fails identically on clean main in this environment (pre-existing, env-dependent).
• Extension: pnpm lint, tsc --noEmit, wxt build, vom:test all clean; vitest +22 new tests green (41 pre-existing React.act failures in tsx files also fail identically on clean main).