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/queries/agent.sql

@@ -74,10 +74,18 @@ RETURNING *;
 
 -- name: CompleteAgentTask :one
 UPDATE agent_task_queue
-SET status = 'completed', completed_at = now(), result = $2
+SET status = 'completed', completed_at = now(), result = $2, session_id = $3, work_dir = $4
 WHERE id = $1 AND status = 'running'
 RETURNING *;
 
+-- name: GetLastTaskSession :one
+-- Returns the session_id and work_dir from the most recent completed task
+-- for a given (agent_id, issue_id) pair, used for session resumption.
+SELECT session_id, work_dir FROM agent_task_queue
+WHERE agent_id = $1 AND issue_id = $2 AND status = 'completed' AND session_id IS NOT NULL
+ORDER BY completed_at DESC
+LIMIT 1;
+
 -- name: FailAgentTask :one
 UPDATE agent_task_queue
 SET status = 'failed', completed_at = now(), error = $2