fix(atlas): allow Sisyphus as last agent when boulder targets atlas explicitly

The boulder continuation in event-handler.ts skipped injection whenever
the last agent was 'sisyphus' and the boulder state had agent='atlas'
set explicitly. The allowSisyphusWhenDefaultAtlas guard required
boulderAgentWasNotExplicitlySet=true, but start-work-hook.ts always
calls createBoulderState(..., 'atlas') which sets the agent explicitly.

This created a chicken-and-egg deadlock: boulder continuation needs
atlas to be the last agent, but the continuation itself is what switches
to atlas. With /start-work, the first iteration was always blocked.

Fix: drop the boulderAgentWasNotExplicitlySet constraint so Sisyphus is
always allowed when the boulder targets atlas (whether explicit or default).

Also reduce todo-continuation-enforcer CONTINUATION_COOLDOWN_MS from
30s to 5s to match atlas hook cooldown and recover interruptions faster.

src/hooks/todo-continuation-enforcer/constants.ts

@@ -17,6 +17,6 @@ export const TOAST_DURATION_MS = 900
 export const COUNTDOWN_GRACE_PERIOD_MS = 500
 
 export const ABORT_WINDOW_MS = 3000
-export const CONTINUATION_COOLDOWN_MS = 30_000
+export const CONTINUATION_COOLDOWN_MS = 5_000
 export const MAX_CONSECUTIVE_FAILURES = 5
 export const FAILURE_RESET_WINDOW_MS = 5 * 60 * 1000