feat(renderers): add keep_thinking option

[samsja]

Summary

• Add a uniform keep_thinking: bool = False parameter to renderers that strip historical <think> blocks from assistant turns before the last user query. When True, prior-turn reasoning is preserved across subsequent user turns.
• Replaces the existing per-renderer knobs preserve_thinking (Qwen3.6) and clear_thinking (GLM-5) for a single clear path, per AGENTS.md guidance.
• Applies to: qwen3, qwen35, qwen36 (inherits), glm5/glm51, glm45, minimax_m2, nemotron3, kimi_k25. Renderers where the concept doesn't apply are untouched (qwen3_vl, kimi_k2, deepseek_v3, gpt_oss, default).

Test plan

• uv run ruff check packages/renderers/ — clean
• uv run pytest packages/renderers/tests/test_render_ids.py — 216 passed (existing parity tests; no regressions)
• uv run pytest packages/renderers/tests/test_keep_thinking.py — 7 passed (new test, parameterized over all 8 affected renderers)
• Re-run full suite in CI (4 pre-existing local failures are tiktoken env issues unrelated to this change)

🤖 Generated with Claude Code

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Note

Medium Risk
Modifies prompt/token rendering across multiple model-specific renderers, which can change model behavior and training masks in multi-turn conversations. Risk is mitigated by defaulting to current behavior (keep_thinking=False) and adding coverage to verify the new mode.

Overview
Adds a uniform keep_thinking: bool = False option to several renderers so callers can preserve prior-turn assistant <think>/reasoning_content blocks across subsequent user turns instead of stripping them by default.

Replaces renderer-specific knobs (clear_thinking in GLM5Renderer, preserve_thinking in Qwen36Renderer) with the shared keep_thinking behavior, and introduces a Qwen3KeepThinkingRenderer plus a new registry name qwen3-keep-thinking.

Adds a new parametrized test (test_keep_thinking.py) that asserts prior-turn reasoning is absent in default renders but present when keep_thinking=True across the affected model families.

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