test(co-browser): Write co-browser test suite

[coderooz]

Write comprehensive tests for the co-browser system.

Acceptance criteria:

• Create tests/co-browser-test.js using node:test runner
• Test window creation and closure
• Test tab claiming and releasing
• Test multi-agent tab locking
• Test instruction panel updates
• Test browser state queries
• Test screenshot capture
• Test navigation via agent
• Test edge cases: double-claim, release by non-owner, close with active tabs
• Test max tabs limit

Test approach:

• Unit tests for CoBrowserManager methods
• Unit tests for InstructionPanel methods
• Mock Playwright for headless testing
• Test tab ownership transfer flow

Depends on: co-browser registration (#49)
Milestone: v2.8.0