feat: add lean PDD prompt mode research harness (#1473)

[prompt-driven-github]

Summary

Adds a publishable research harness (research/lean-prompt-mode/) that measures and reduces PDD prompt token overhead through deterministic section-level pruning, while preserving all required source-truth, output, verifier, delimiter, and safety contract fields. Emits auditable prompt_policy.json and prompt_token_audit.json artifacts on every lean run.

Closes #1473

Changes Made

New Research Harness Files

• research/lean-prompt-mode/lean_prompt.py — main pruner with --mode {current,lean} parameter; emits both audit artifacts; uses litellm/tiktoken for token counting
• research/lean-prompt-mode/README.md — design document, usage guide, fixture format, CI integration notes
• research/lean-prompt-mode/fixtures/current_prompt.txt — frozen golden PDD prompt in current (unmodified) form
• research/lean-prompt-mode/fixtures/lean_prompt.txt — expected lean-pruner output on the golden fixture
• research/lean-prompt-mode/fixtures/prompt_policy.json — expected prompt_policy.json for the golden fixture
• research/lean-prompt-mode/fixtures/prompt_token_audit.json — expected prompt_token_audit.json for the golden fixture
• research/lean-prompt-mode/schemas/prompt_policy.schema.json — JSON Schema v1 for prompt_policy.json
• research/lean-prompt-mode/schemas/prompt_token_audit.schema.json — JSON Schema v1 for prompt_token_audit.json
• research/lean-prompt-mode/tests/test_lean_prompt.py — unit tests: pruning rules, invariant preservation, schema validation, default-mode regression, anomaly detection

Documentation Updated

• docs/prompting_guide.md — new ## Lean Prompt Mode section added after ## Automated Context Compression, covering pruning rules, audit artifacts, structural floor, relationship to context compression, and default behavior
• README.md — new ### Lean Prompt Mode Research Harness subsection under ## Utilities after ### Regenerate Model Catalog

Review Checklist

• Prompt syntax is valid
• PDD conventions followed
• Documentation is up to date
• lean_prompt.py preserves all five required field groups (source-truth content, output files, verifier command, output delimiters, safety constraints)
• prompt_policy.json and prompt_token_audit.json schema versions are "1.0"
• Default mode (--mode current) leaves prompts unchanged
• Tests pass: pytest research/lean-prompt-mode/tests/

Next Steps After Merge

1. No modules to sync (research harness is additive; no PDD-managed .prompt files modified)
2. Run tests to verify:

   pytest research/lean-prompt-mode/tests/ -vv

3. Try the harness on a real prompt:

   python research/lean-prompt-mode/lean_prompt.py --mode lean prompts/your_module.prompt

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Created by pdd change workflow