docs: clarify browser session observability

[Tomus2b2a]

Summary

• clarify that tab list reflects the named browser lease, not a global inventory of every ordinary Chrome tab
• document that a new owned session may begin on about:blank
• require URL/state verification after bind, with unbind/retry when the wrong origin was leased

Validation

• skills/opencli-usage and skills/opencli-browser pass the skill validator
• git diff --check

These clarifications come from dogfooding the current browser bridge against a connected Chrome profile.