Record a Browserbase session that was opened by the @browserbasehq/mcp server (i.e. the agent called a browser tool, not the explicit maritime-request-login flow).

Called by the maritime-bb-mcp-wrap shell wrapper around the bb-mcp binary, which tails stderr for the SessionManager's "Browserbase Live Debugger URL: .../sessions/<id>" line and POSTs each session id here. Inserts a browser_sessions row so the dashboard's Live Browser panel (which polls /api/agents/{id}/browser/live-view and reads from that table) auto-expands for MCP-driven browsing, not only for the maritime-request-login path.

Idempotent: record_session_start no-ops on duplicate session_id, so retries (wrapper restart, transient curl error) don't double-count.

Session billing close still goes through get_live_view's lazy reap (which marks ended_at but does NOT call bill_session_close, the same gap as the request-login path; tracked separately).