feat(co-browser): Create co-browser manager with shared window and tab locking

[coderooz]

Create the core co-browser manager with shared Playwright browser window, tab ownership, and multi-agent coordination.

Acceptance criteria:

• Create src/domains/browser/co-browser.js with CoBrowserManager class
• Shared Playwright BrowserContext per window (headless=false for user interaction)
• Tab ownership model: each tab locked to one agent
• Tab claiming/releasing with agent validation
• Navigation proxy: agent navigates tab via Playwright, user sees live page
• Screenshot capture for SSE viewport streaming
• Window state query (all tabs, owners, URLs, instructions)
• Emergency tab release on agent heartbeat failure
• Max tabs per window: configurable (default: 10)

Class interface:

• openWindow(agentId, windowName) → { windowId, tabId }
• addTab(windowId, agentId) → { tabId }
• claimTab(windowId, tabId, agentId) → success
• releaseTab(windowId, tabId, agentId) → success
• navigateTab(windowId, tabId, url) → { url, title }
• getScreenshot(windowId, tabId) → base64 image
• getState(windowId) → full state
• closeWindow(windowId, agentId) → success

Milestone: v2.8.0