fix: implement deriver custom instructions

[adavyas]

Summary

Implements reasoning.custom_instructions for the deriver on top of the prefix-cache prompt split from #425.

What Changed

• keeps custom_instructions in resolved reasoning config
• adds custom_instructions to the deriver user prompt
• updates deriver prompt token estimation to include optional custom instructions
• adds tests for config precedence, prompt rendering, and deriver wiring

Scope

This only implements custom_instructions for the deriver.

It does not add custom_instructions support to:

• dialectic
• dreamer

Why

The schema already exposed reasoning.custom_instructions, but it was not used at runtime. This wires it into the deriver while preserving the cache-friendly split introduced in #425 by keeping the instructions in the user message rather than the stable system prompt.

Testing

Verified locally with:

• python -m py_compile on changed files
• basedpyright on changed files
• targeted pytest on:
  • tests/deriver/test_deriver_processing.py
  • tests/deriver/test_prompts.py
  • tests/utils/test_config_helpers.py

Targeted pytest result:

• 15 passed

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