fix: skill loading should default to 1000 lines (#459)

## Description
Ports the skill loading fix from the Python SDK
(langchain-ai/deepagents#2721) to JS. The system prompt now instructs
agents to pass `limit=1000` when reading skill files via `read_file`,
since the default of 100 lines is too small for most skill files.

## Test Plan
- [ ] Verify skill loading prompt includes `limit=1000` instruction

_Opened collaboratively by Sydney Runkle and open-swe._

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Co-authored-by: open-swe[bot] <open-swe@users.noreply.github.com>
Co-authored-by: Sydney Runkle <54324534+sydney-runkle@users.noreply.github.com>
Co-authored-by: Hunter Lovell <40191806+hntrl@users.noreply.github.com>