feat(agent): add per-task session persistence for Claude Code resumption

Store the Claude Code session ID and working directory when a task
completes. On the next task for the same (agent, issue) pair, look up
the prior session and pass --resume <session_id> to Claude Code so
the agent retains conversation context across multiple tasks on the
same issue.

Changes:
- Migration 020: add session_id and work_dir columns to agent_task_queue
- CompleteAgentTask stores session_id and work_dir on completion
- GetLastTaskSession query retrieves prior session for (agent, issue)
- ClaimTaskByRuntime handler populates prior_session_id in response
- Daemon passes ResumeSessionID through to Claude backend Execute()
- Claude backend adds --resume flag when ResumeSessionID is set

server/migrations/020_task_session.down.sql

@@ -0,0 +1,2 @@
+ALTER TABLE agent_task_queue DROP COLUMN IF EXISTS session_id;
+ALTER TABLE agent_task_queue DROP COLUMN IF EXISTS work_dir;