fix(delegation): harden subagents against truncated tool-call output

[stefan-mcf]

Summary

• add the missing public config/docs note for delegation.max_tokens
• document that child subagents can need a larger output-token budget than the parent
• explain that this knob is for Hermes-side truncated tool-call recovery, not for changing user prompting expectations

Why

This PR fixes a Hermes-side delegation robustness issue. When a subagent hits finish_reason="length" while emitting tool-call arguments, Hermes correctly refuses to execute incomplete arguments, but operators need a documented child-specific output budget override to prevent repeated failures in code-edit-heavy lanes.

This is not caused by incorrect user prompting. It is a Hermes-side hardening gap around truncated subagent tool-call output.

Test plan

• python -m pytest tests/tools/test_delegate.py -q -k 'delegation_max_tokens or prompt_tells_subagents_to_avoid_oversized_tool_calls'
• python -m pytest tests/run_agent/test_run_agent.py -q -k 'truncated_tool_call'

[stefan-mcf]

Closing and replacing with two narrower PRs for cleaner review: the delegation/subagent truncation hardening remains in this PR branch history but is now updated as a focused delegation-only PR, and the separate compression continuity advisory change now lives in #21118.