feat: add two-level work mode (projectExecution + roleExecution) (#15) (#18)

Replaces single 'workMode' with two distinct settings:

1. Plugin-level 'projectExecution' (parallel | sequential)
   - parallel: each project can have active workers independently
   - sequential: only one project can have active workers at a time

2. Project-level 'roleExecution' (parallel | sequential)
   - parallel: DEV and QA can run simultaneously on same project
   - sequential: only one role (DEV or QA) active at a time per project

Changes:
- index.ts: rename workMode → projectExecution in config schema
- projects.ts: add roleExecution field to Project type
- heartbeat_tick: check both levels before picking up tasks
- task_pickup: enforce roleExecution when picking up manually
- project_register: accept roleExecution param (default: parallel)
- devclaw_setup: accept projectExecution param

All defaults remain 'parallel' for backward compatibility.

lib/tools/task-pickup.ts

@@ -229,6 +229,18 @@ export function createTaskPickupTool(api: OpenClawPluginApi) {
         );
       }
 
+      // 6b. Check project-level roleExecution
+      const roleExecution = project.roleExecution ?? "parallel";
+      if (roleExecution === "sequential") {
+        const otherRole = role === "dev" ? "qa" : "dev";
+        const otherWorker = getWorker(project, otherRole);
+        if (otherWorker.active) {
+          throw new Error(
+            `Project "${project.name}" has sequential roleExecution: ${otherRole.toUpperCase()} worker is active (issue: ${otherWorker.issueId}). Wait for it to complete first.`,
+          );
+        }
+      }
+
       // 7. Select model (priority: param > tier label > heuristic)
       const targetLabel: StateLabel = role === "dev" ? "Doing" : "Testing";
       let modelAlias: string;