feat(daemon): reuse workdir across tasks on same agent+issue pair

Previously each task created a fresh workdir via execenv.Prepare(), even
when resuming work on the same (agent, issue). This caused the agent's
session context to be out of sync with a blank code state.

Now the server returns prior_work_dir in the claim response, and the
daemon tries execenv.Reuse() first — which wraps the existing directory,
detects git worktree state, and refreshes context files. Falls back to
Prepare() if the prior workdir no longer exists. Workdirs are no longer
cleaned up after task completion so they remain available for reuse.

server/internal/handler/agent.go

@@ -96,6 +96,7 @@ type AgentTaskResponse struct {
 	Agent          *TaskAgentData `json:"agent,omitempty"`
 	CreatedAt      string         `json:"created_at"`
 	PriorSessionID string         `json:"prior_session_id,omitempty"` // session ID from a previous task on same issue
+	PriorWorkDir   string         `json:"prior_work_dir,omitempty"`   // work_dir from a previous task on same issue
 }
 
 // TaskAgentData holds agent info included in claim responses so the daemon

server/internal/handler/daemon.go

@@ -224,6 +224,9 @@ func (h *Handler) ClaimTaskByRuntime(w http.ResponseWriter, r *http.Request) {
 		IssueID: task.IssueID,
 	}); err == nil && prior.SessionID.Valid {
 		resp.PriorSessionID = prior.SessionID.String
+		if prior.WorkDir.Valid {
+			resp.PriorWorkDir = prior.WorkDir.String
+		}
 	}
 
 	slog.Info("task claimed by runtime", "task_id", uuidToString(task.ID), "runtime_id", runtimeID, "agent_id", uuidToString(task.AgentID), "prior_session", resp.PriorSessionID)