By default (inherit_system_prompt=False) the sub-agent got only the 17-line
subagent_system_prompt — none of the parent's editing/reading/context
discipline. So a sub-agent with a clean context would still burn it: pulling
whole files into read, over-using smart_edit, not tracking steps. The clean
context is the sub-agent's main advantage — keep it clean.

Expand subagent_system_prompt.txt with a compact operational core:
- Editing: prefer edit/edit_lines (deterministic); smart_edit as last resort
  (extra LLM call, reads the whole file) — the same policy the parent now has.
- Reading: info before read, offset/limit to the region, grep/find/query to
  locate — don't read a file just to search it.
- Track steps with todo and record findings in scratchpad (durable within the
  run; context can be compressed).

inherit_system_prompt stays False: the full parent prompt carries orchestration
and spawn_agent sections that are irrelevant to a sub-agent and references
tools it lacks. The sub-agent keeps its own focused, self-contained prompt.

Co-Authored-By: Claude <noreply@anthropic.com>