Support DSH /plan mode without explicit request mode

[cty41]

Problem

DeepSeek Harness keeps /plan as command-plane session state. Its OpenAI-compatible requests do not send mode or X-Cursor-Mode, so the proxy falls back to its configured default (normally ask) even though DSH injects a stable plan policy into trusted system content.

Proposed behavior

Add an opt-in DSH auto-mode bridge with precedence:

1. request body mode
2. X-Cursor-Mode
3. inferred DSH mode
4. configured default

Only inspect system/developer content. Identify DSH through its stable system preamble, select plan when You are in plan mode. is present, and select agent for identified DSH requests when it is absent. Never infer from ordinary user text.

This keeps existing clients unchanged and lets /plan off return naturally to Agent mode.