🤖 feat: add code snippet guidance to Plan Mode

[ibetitsmike]

Summary

• Encourage Plan Mode plans to include reasonably sized code snippets when listing implementation details.

Background

• Plans often include only prose, which can leave implementation steps ambiguous. This adds explicit guidance to include small, focused snippets for key changes.

Implementation

• Updated shared Plan Mode instruction string to recommend focused code snippets in implementation details.
• Updated the built-in Plan agent prompt (and generated docs/agent index) to include an “Implementation details” bullet with the same guidance.

Validation

• make fmt
• make static-check

Risks

• Low: prompt-only change; no runtime logic.

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Generated with mux • Model: openai:gpt-5.2 • Thinking: xhigh • Cost: N/A