Fix Claude usage-limit prose handling

[hniska-identione]

Context

Claude usage-limit prose was treated as a clean turn, causing IDE-162 to loop until max turns.

TL;DR

Classify Claude "hit your limit" prose as a rate-limit error.

Summary

• Detect Claude usage-limit reset notices even when AssistantMessage.error is unset.
• Emit a structured rate_limit error with the existing retry-after hint.
• Add regression coverage for the June 17 trace shape.

Alternatives

• Wait for SDK error fields only; rejected because the observed trace had plain assistant prose.

Test Plan

• make -C elixir all
• cd elixir && uv run --project priv/claude_agent pytest priv/claude_agent/tests/test_forward_message_rate_limit.py