Bounded browser sessions

Added

• Bounded Browserbase management for the single-user dispatcher: one active workflow, two FIFO waiters, a 30-second queue wait, a three-minute session lifetime, and idle cleanup. Admission uses the existing transactional store, and unconfirmed releases remain fenced.
• Read-only browser_status MCP inspection and ifd-simbrief sessions / cleanup commands. Cleanup targets only dispatcher-owned sessions older than the session budget.
• Queue, deadline, release-confirmation, and minimum-adjusted browser-minute telemetry.

Changed

• Browserbase requests use short timeouts without automatic retries. Browser commands and cleanup are bounded, disconnected sessions end automatically, and timed-out workflows do not open replacement browsers. The daily keepalive route skips browser work; login refresh happens on demand. Remote admission limits outstanding agent turns to six.

Local checks and isolated Cloudflare staging acceptance pass. Two retained A20N plans used two non-overlapping browser sessions and increased reported usage by two minutes. See live acceptance.