fix(taskctl): clear assignee on developing→reviewing transition and add happy path integration test

[randomm]

Fixes #257

Root Cause

heartbeatActiveAgents was not clearing assignee and assignee_pid when transitioning a task from stage: developing to stage: reviewing. The spawnAdversarial filter requires !t.assignee — so the adversarial agent was never spawned, leaving tasks stuck at developing forever.

Fix

• heartbeatActiveAgents: clear assignee: null, assignee_pid: null on developing→reviewing transition
• Export heartbeatActiveAgents and scheduleReadyTasks for test access
• Add allowImmutable: true to Store.updateTask calls that modify pipeline field

Test

New integration test pulse-happy-path.test.ts drives the complete flow:
scheduleReadyTasks → developing → heartbeat → reviewing → spawnAdversarial → APPROVED → done