Require approval before opening browser URLs

[CooperSheroy]

Summary

• classify browser_open_url as gate-worthy so model-supplied outbound navigation prompts before opening a real browser URL
• keep browser page reads such as browser_snapshot free
• update connector approval tests to cover navigation gating

Fixes #399.

Tests

• uv run --extra dev pytest tests/test_send_target_resolution.py tests/test_url_address_guard.py tests/test_connectors.py::test_engine_connector_tools_are_cowork_scoped -q
• git diff --check