feat(co-browser): Create co-browser MCP tool handlers

[coderooz]

Create MCP tool handlers for the co-browser system.

Acceptance criteria:

• Create tools/coBrowserTools.js
• Implement open_shared_browser handler
• Implement set_instructions handler
• Implement claim_tab handler
• Implement release_tab handler
• Implement get_browser_state handler
• Implement close_shared_browser handler
• Follow existing tool handler pattern

Tool definitions:

• open_shared_browser: { agent_id, window_name? } → { windowId, tabId, viewportUrl }
• set_instructions: { windowId, instructions, tabId? } → success
• claim_tab: { windowId, tabId, agent_id } → success
• release_tab: { windowId, tabId, agent_id } → success
• get_browser_state: { windowId } → { tabs, instructions, activeTab }
• close_shared_browser: { windowId, agent_id } → success

Depends on: co-browser manager (#45), instruction panel (#46)
Milestone: v2.8.0