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/projects.ts

@@ -23,6 +23,8 @@ export type Project = {
   baseBranch: string;
   deployBranch: string;
   autoChain: boolean;
+  /** Project-level role execution: parallel (DEV+QA can run simultaneously) or sequential (only one role at a time). Default: parallel */
+  roleExecution?: "parallel" | "sequential";
   maxDevWorkers?: number;
   maxQaWorkers?: number;
   dev: WorkerState;