feat: wire worker scratchpad to shared supervisor thread

- Invert thread_id resolution priority in ScratchpadMixin._execute_scratchpad_logic
  so self._scratch_pad_thread wins over positional thread_id arg, ensuring worker
  scratchpad calls route to the supervisor's Redis key not the worker's own thread

- Set self._scratch_pad_thread explicitly in DelegationMixin.handle_delegate_research_task
  to guarantee the value is present for the duration of each delegation cycle

- Rewrite RESEARCH_WORKERS_INSTRUCTIONS scratchpad blocks:
  - L4_WORKER_IDENTITY: reframe scratchpad append as primary deliverable
  - L4_WORKER_SCRATCHPAD_PROTOCOL: add collision avoidance, VERIFIED short-circuit,
    tombstone awareness, and no-duplication rule between scratchpad and text reply
  - L4_EXECUTION_ALGORITHM: integrate scratchpad steps into algorithm as
    non-negotiable checkpoints rather than advisory side-effects
  - L4_TOOL_CHEATSHEET: demote to reference card only
  - L4_PARALLEL_EXECUTION: tighten parallel first strike rules
  - L4_STOPPING_CRITERIA: add already-verified short-circuit condition
  - Remove L4_DEPTH_PROTOCOL from dict (key retained in include list pending review)

Author: Francisco