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.

index.ts

@@ -30,10 +30,10 @@ const plugin = {
           qa: { type: "string", description: "QA engineer model" },
         },
       },
-      workMode: {
+      projectExecution: {
         type: "string",
         enum: ["parallel", "sequential"],
-        description: "Work mode: parallel (each project independent) or sequential (1 DEV + 1 QA globally)",
+        description: "Plugin-level project execution: parallel (each project independent) or sequential (only one project active at a time)",
         default: "parallel",
       },
       orchestratorDm: {