fix(claude_backend): translate effort to claude CLI --thinking enum

[cyberxuan-XBX]

Summary

The Claude CLI's --thinking flag accepts only the enum values enabled, adaptive, or disabled, but skillopt/model/claude_backend.py forwarded the OpenAI-style low / medium / high effort values unchanged. As a result, every claude_chat rollout failed with:

error: option '--thinking <mode>' argument 'low' is invalid.
Allowed choices are enabled, adaptive, disabled.

This made the claude_chat backend unusable on the current Claude CLI (verified on 2.1.154).

Change

• Add _OPENAI_TO_CLAUDE_CLI_THINKING mapping (low → disabled, medium → adaptive, high|xhigh|max → enabled).
• Extend _normalize_reasoning_effort to also accept the CLI's native enum values (enabled / adaptive / disabled) so users can opt in to a specific mode directly.
• Introduce _claude_cli_thinking_value and use it at the only --thinking call site in _run_claude_print.
• Unknown values continue to silently drop the flag, matching the prior behavior.

Verification

Reproduced the original failure with a small SearchQA split using claude-sonnet-4-6:

• Before: every rollout returned agent_ok=False with Claude backend failed after 5 retries: error: option '--thinking <mode>' argument 'low' is invalid.
• After: with reasoning_effort: low, rollouts succeed (agent_ok=True), and a 2-step / 8-train-item / 4-val toy run completes 26 calls in 76s, with completion tokens dropping from ~4k (no thinking) to ~2k (now correctly translated to --thinking disabled).

No additional dependencies. Diff is +26 / -3 in a single file. Existing OpenAI-style usage (low / medium / high) keeps working transparently.

Notes

• The mapping is intentionally conservative: anything outside the documented OpenAI levels and the CLI enum is dropped rather than guessed at, so we don't accidentally change behavior for unrecognized values.
• Happy to add a brief unit test if there's a preferred location — I didn't see a tests/ directory in the tree, so I held off to avoid introducing a new convention.

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