fix(prompt): calibrate response length to question scope to reduce verbosity

[langsmith-engine-dev]

Summary

• Root cause: docs_agent_prompt.py instructs the agent to always "show working examples — copy-paste ready code" with no guidance on calibrating response length for follow-up clarifications, incremental change requests, or simple yes/no questions. This drives a 90% failure rate on the properly-terse-answer metric (18+ failures in a 100-trace sample).
• Fix: Added a "Calibrate Response Length to Question Scope" section in the Writing Rules and corresponding DO/DON'T bullets that distinguish between full how-to questions (comprehensive answer appropriate) vs. follow-ups/incremental changes (show only the delta, be brief).
• Scope: Minimal targeted addition to src/prompts/docs_agent_prompt.py — no code changes, no test changes.

Evidence traces (production)

All traces from the Chat-LangChain project showing overly-verbose responses:

• User says "再添加个word文档" (add Word doc support) → got 3843-char full code rewrite
• User asks how to switch to Redis → got 6110-char full implementation
• User gives brief chart-type clarification → got full explanation with install instructions
• User clarifies about planner prompt → got 2167-char answer
• Version pinning complaint → got "ultimate solution" full rewrite
• Direct question → got too-long answer with sections and bullets
• Handoff question → got a very long answer

Test plan

• Existing unit tests pass (uv run pytest tests/unit/ -q — 7 passed, pre-existing failures in test_broken_doc_urls.py and test_check_links_async.py are unrelated)
• No existing open PR covers this root cause
• Change is minimal and scoped to the verbosity problem only
• Monitor properly-terse-answer metric on next production trace batch

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