Inline deviation notes into bundled practitioner guide

The guide's opening blockquote previously pointed at
`docs/methodology/REGISTRY.md` for details on reorganizations relative to
Baker et al. (2025). That path does not exist inside a wheel-only install,
so it was a dead breadcrumb for offline AI agents - exactly the scenario
this PR is meant to fix.

Inline the four deviation bullets (numbering, PT as Step 3, uncertainty
folding into Step 5, Step 8 = Robustness & Reporting) directly into the
guide so it is self-contained.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: igerber

diff_diff/guides/llms-practitioner.txt

@@ -3,7 +3,21 @@
 > An 8-step workflow for rigorous Difference-in-Differences analysis, based on
 > Baker et al. (2025) "Difference-in-Differences Designs: A Practitioner's
 > Guide" and adapted for the diff-diff library. Some steps are reorganized or
-> extended relative to the paper (see docs/methodology/REGISTRY.md for details).
+> extended relative to the paper:
+>
+> - **Numbering**: diff-diff uses 1-Define, 2-Assumptions, 3-Test PT,
+>   4-Choose estimator, 5-Estimate, 6-Sensitivity, 7-Heterogeneity,
+>   8-Robustness. The paper uses 1-Define, 2-Assumptions, 3-Estimation method,
+>   4-Uncertainty, 5-Estimate, 6-Sensitivity, 7-Heterogeneity, 8-Keep learning.
+> - **Parallel trends testing** is a separate Step 3 (the paper embeds it in
+>   Step 2), to ensure AI agents execute it as a distinct action.
+> - **Sources of uncertainty** (paper's Step 4) are folded into Step 5
+>   (Estimate) with an explicit cluster-count check directive: >= 50 clusters
+>   for asymptotic SEs, otherwise wild bootstrap. The 50-cluster threshold is
+>   a diff-diff convention.
+> - **Step 8** is "Robustness & Reporting" (compare estimators, report with
+>   and without covariates). The paper's Step 8 is "Keep learning." The
+>   mandatory with/without covariate comparison is a diff-diff convention.
 
 ## Instructions for AI Agents