* fix(gh-monitor): notify main session immediately on new PR activity
Previously, dispatch_agent only fired an isolated agent to handle the event.
The main session (Hans) was never directly notified, so new PR comments
only surfaced when Andrew messaged directly.
Now calls notify() immediately before dispatching the agent, so the main
session gets a system event about the new activity in real-time.
* refactor(gh-monitor): route PR activity through main session instead of isolated agents
Replaced dispatch_agent() (which spawned isolated cron jobs) with
dispatch_to_main() (which sends a system event to the main session).
This way Hans handles PR comments directly in the main session —
no sync issues between isolated and main sessions, no lost context.
