Add AI agent discoverability for practitioner guide

[igerber]

Summary

• Add "For AI Agents" section to README.md pointing to docs/llms.txt and docs/llms-practitioner.txt
• Update package docstring (help(diff_diff)) to mention the 8-step practitioner workflow and practitioner_next_steps()
• Add "Practitioner Guide" URL to PyPI project metadata in pyproject.toml

Methodology references (required if estimator / math changes)

• Method name(s): N/A — no methodology changes
• Paper / source link(s): N/A
• Any intentional deviations from the source (and why): None

Validation

• Tests added/updated: No test changes (documentation/metadata only)
• Backtest / simulation / notebook evidence (if applicable): N/A

Security / privacy

• Confirm no secrets/PII in this PR: Yes

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[github-actions]

Overall Assessment

✅ Looks good

Executive Summary

• No estimator, weighting, variance/SE, identification, or default-behavior code changed in this PR, so the methodology-risk surface is low.
• The new Baker et al. references are consistent with the documented Practitioner Guide adaptation in docs/methodology/REGISTRY.md:L2116-L2131; there is no methodology defect here.
• P2: The new package docstring in diff_diff/init.py:L7-L12 points users to repo-relative docs/ files that are not shipped in the installed wheel; tool.maturin only packages diff_diff in pyproject.toml:L94.
• P2: The new PyPI metadata link in pyproject.toml:L77-L82 targets blob/main, so release metadata can drift away from the specific version the user installed.
• P3 informational, already tracked: .txt AI guides remain outside doc-snippet CI coverage in TODO.md:L83.

Methodology

• P3 Informational. Impact: The new wording in README.md:L70-L76 and diff_diff/init.py:L7-L12 says the workflow is “based on Baker et al. 2025.” That is acceptable because the deviations are already documented under **Note:** labels in docs/methodology/REGISTRY.md:L2116-L2131. Concrete fix: None required. Optional wording cleanup: use “adapted from” to match the registry exactly.

Code Quality

No findings in the changed code.

Performance

No findings. The PR is documentation/metadata only.

Maintainability

• P2. Impact: The new Practitioner Guide project URL in pyproject.toml:L77-L82 points to github.com/.../blob/main/..., which is a moving target. A user on a released version can be sent to guide text from a later commit, creating version skew between installed behavior and linked instructions. Concrete fix: point this URL to a versioned/stable location, preferably a published docs URL or a tag-pinned GitHub URL.

Tech Debt

• P3 Informational, tracked in TODO. Impact: AI guide files are still outside smoke-test coverage in TODO.md:L83. This PR increases discoverability of those files, but the gap is already tracked, so it is not a blocker. Concrete fix: extend tests/test_doc_snippets.py to scan .txt guides, or publish the guide through the normal docs pipeline.

Security

No findings. No secrets, credentials, or new execution surfaces are introduced.

Documentation/Tests

• P2. Impact: The new module docstring in diff_diff/init.py:L7-L12 tells users running help(diff_diff) to read docs/llms-practitioner.txt and docs/llms.txt, but the distribution only packages diff_diff itself in pyproject.toml:L94. In a normal wheel install, those repo-relative paths will not exist, so the new discoverability path breaks in the installed-package context this change is targeting. Concrete fix: replace repo-relative paths in the package docstring with a stable public URL, or ship the guide files as package data and resolve them via package resources.