Add AI agent discoverability for practitioner guide

Three discovery surfaces updated so agents find the practitioner
workflow regardless of how they encounter the library:

1. Package docstring (help(diff_diff)) — now mentions the 8-step
   practitioner workflow and practitioner_next_steps()
2. README.md — new "For AI Agents" section after Quick Start,
   pointing to docs/llms.txt and docs/llms-practitioner.txt
3. pyproject.toml — "Practitioner Guide" URL in project metadata,
   visible on PyPI sidebar

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: igerber

README.md

@@ -67,6 +67,14 @@ Signif. codes: '***' 0.001, '**' 0.01, '*' 0.05, '.' 0.1
 ======================================================================
 ```
 
+## For AI Agents
+
+If you are an AI agent or LLM using this library, read [`docs/llms.txt`](docs/llms.txt) for a concise API reference with an 8-step practitioner workflow (based on Baker et al. 2025). The workflow ensures rigorous DiD analysis — not just calling `fit()`, but testing assumptions, running sensitivity analysis, and checking robustness.
+
+After estimation, call `practitioner_next_steps(results)` for context-aware guidance on remaining diagnostic steps.
+
+Detailed guide: [`docs/llms-practitioner.txt`](docs/llms-practitioner.txt)
+
 ## Features
 
 - **sklearn-like API**: Familiar `fit()` interface with `get_params()` and `set_params()`

diff_diff/__init__.py

@@ -3,6 +3,13 @@
 
 This library provides sklearn-like estimators for causal inference
 using the difference-in-differences methodology.
+
+For rigorous analysis, follow the 8-step practitioner workflow in
+docs/llms-practitioner.txt (based on Baker et al. 2025). After
+estimation, call ``practitioner_next_steps(results)`` for context-aware
+guidance on remaining diagnostic steps.
+
+AI agent reference: docs/llms.txt
 """
 
 # Import backend detection from dedicated module (avoids circular imports)

pyproject.toml

@@ -79,6 +79,7 @@ Homepage = "https://github.com/igerber/diff-diff"
 Documentation = "https://diff-diff.readthedocs.io"
 Repository = "https://github.com/igerber/diff-diff"
 Issues = "https://github.com/igerber/diff-diff/issues"
+"Practitioner Guide" = "https://github.com/igerber/diff-diff/blob/main/docs/llms-practitioner.txt"
 
 [tool.maturin]
 # Build the Rust extension module