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/agent/agent.go

@@ -20,11 +20,12 @@ type Backend interface {
 
 // ExecOptions configures a single execution.
 type ExecOptions struct {
-	Cwd          string
-	Model        string
-	SystemPrompt string
-	MaxTurns     int
-	Timeout      time.Duration
+	Cwd             string
+	Model           string
+	SystemPrompt    string
+	MaxTurns        int
+	Timeout         time.Duration
+	ResumeSessionID string // if non-empty, resume a previous Claude Code session
 }
 
 // Session represents a running agent execution.

server/pkg/agent/claude.go

@@ -47,6 +47,9 @@ func (b *claudeBackend) Execute(ctx context.Context, prompt string, opts ExecOpt
 	if opts.SystemPrompt != "" {
 		args = append(args, "--append-system-prompt", opts.SystemPrompt)
 	}
+	if opts.ResumeSessionID != "" {
+		args = append(args, "--resume", opts.ResumeSessionID)
+	}
 	args = append(args, "-p", prompt)
 
 	cmd := exec.CommandContext(runCtx, execPath, args...)