feat(review): parallel execution warning on flowctl start

When starting a task while other tasks in the same epic are already
in_progress, flowctl now emits:
- stderr warning about worktree isolation requirement
- JSON field "parallel_warning" with concurrent task count

This catches the #1 Critical audit finding (3 consecutive rounds):
workers running in the same directory without worktree isolation.

The warning is informational (not blocking) because flowctl cannot
control the Agent tool's isolation parameter. But the message appears
in the agent's tool output, making it harder to ignore.

Use --force to suppress the warning.

370 tests pass.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: z23cc

bin/flowctl

@@ -17,6 +17,31 @@ pub fn cmd_start(json_mode: bool, id: String, force: bool, _note: Option<String>
     let flow_dir = ensure_flow_exists();
     let actor = resolve_actor();
 
+    // ── Parallel execution safety check ────────────────────────
+    // If other tasks in the same epic are already in_progress, warn about
+    // worktree isolation requirement. This catches the #1 audit failure:
+    // workers running in the same directory without isolation.
+    let epic_id = flowctl_core::id::epic_id_from_task(&id).unwrap_or_default();
+    if !epic_id.is_empty() {
+        if let Ok(tasks) = flowctl_core::json_store::task_list_by_epic(&flow_dir, &epic_id) {
+            let in_progress: Vec<String> = tasks
+                .iter()
+                .filter(|t| t.status == flowctl_core::state_machine::Status::InProgress && t.id != id)
+                .map(|t| t.id.clone())
+                .collect();
+            if !in_progress.is_empty() && !force {
+                eprintln!(
+                    "⚠️  PARALLEL EXECUTION WARNING: {} other task(s) already in_progress: {}\n\
+                     Workers MUST use isolation: \"worktree\" to prevent race conditions.\n\
+                     If workers share the same directory, file edits will conflict.\n\
+                     Use --force to suppress this warning.",
+                    in_progress.len(),
+                    in_progress.join(", ")
+                );
+            }
+        }
+    }
+
     let req = StartTaskRequest {
         task_id: id.clone(),
         force,
@@ -26,11 +51,27 @@ pub fn cmd_start(json_mode: bool, id: String, force: bool, _note: Option<String>
     match flowctl_core::lifecycle::start_task(&flow_dir, req) {
         Ok(resp) => {
             if json_mode {
-                json_output(json!({
+                let mut out = json!({
                     "id": resp.task_id,
                     "status": "in_progress",
-                    "message": format!("Task {} started", resp.task_id),
-                }));
+                });
+                // Include parallel warning in JSON so agent sees it
+                let epic_id = flowctl_core::id::epic_id_from_task(&resp.task_id).unwrap_or_default();
+                if !epic_id.is_empty() {
+                    if let Ok(tasks) = flowctl_core::json_store::task_list_by_epic(&flow_dir, &epic_id) {
+                        let in_progress_count = tasks
+                            .iter()
+                            .filter(|t| t.status == flowctl_core::state_machine::Status::InProgress)
+                            .count();
+                        if in_progress_count > 1 {
+                            out["parallel_warning"] = json!(format!(
+                                "{} tasks now in_progress — use isolation:worktree for each worker",
+                                in_progress_count
+                            ));
+                        }
+                    }
+                }
+                json_output(out);
             } else {
                 println!("Task {} started", resp.task_id);
             }