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/pkg/db/generated/models.go

@@ -73,6 +73,8 @@ type AgentTaskQueue struct {
 	CreatedAt    pgtype.Timestamptz `json:"created_at"`
 	Context      []byte             `json:"context"`
 	RuntimeID    pgtype.UUID        `json:"runtime_id"`
+	SessionID    pgtype.Text        `json:"session_id"`
+	WorkDir      pgtype.Text        `json:"work_dir"`
 }
 
 type Comment struct {