Run Operator ambient loop only while fleet active

[edwin-zvs]

Summary

• Change the Operator ambient loop default cadence from 5 minutes to 60 seconds.
• Gate ambient ticks on fleet activity: every interval, Operator checks for an active non-Operator, non-terminal session and only emits the ambient tick when one exists.
• Treat recent PTY activity, recent transcript/event activity, and brand-new running sessions without timestamps as active.
• Stop ticking when all sessions are idle/terminal/inactive instead of falling back to a 5-minute idle loop.

Tests

• cargo test -p agentd-adapter-zarvis ambient
• cargo build
• git diff --check