docs(rag): 0.7.0 spec — path-keyed sidecars + Shape 2 validation (#1)

Adds two planning documents:

1. path-keyed-sidecars-for-rag-v0.7.0.md — draft spec for
   attune-help 0.7.0 that ships path-keyed variants of
   summaries.json and cross_links.json, so attune-rag's
   DirectoryCorpus can actually use the summary/cross-link
   signal (current sidecars are feature-keyed and silently
   ignored by path-keyed consumers). Includes the LLM
   polish prompt, 5-task breakdown, alternatives, and
   sequencing notes.

2. shape2-validation-bug-predict-2026-04-18.md — empirical
   validation of the spec's thesis on a single feature
   (bug-predict). Hand-crafted keyword-rich path-keyed
   summaries lifted fixture-benchmark Precision@1 from
   36% to 76% (+40 pts) without changing the retriever.
   Confirms path-keyed summaries are the highest-leverage
   RAG improvement in the roadmap.

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

Author: silversurfer562