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.
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