fix(taskctl): use assignee_pid for liveness in resurrectionScan and heartbeatActiveAgents

[randomm]

Fixes #252, #253, #254

Problem

After binary restart, SessionStatus (module-level in-memory Map) retains stale busy entries. Both resurrectionScan and heartbeatActiveAgents trusted it exclusively, causing tasks stuck at stage: developing forever.

Fix

• resurrectionScan: alive = pidAlive || sessionAlive. stage=developing + dead -> advance to reviewing, clear assignee, preserve worktree/branch. Other stages -> reset to idle.
• heartbeatActiveAgents: alive = sessionAlive && pidAlive. Default pidAlive=true when no assignee_pid.