docs: drop docs/INDEX.md, add PRD README with format

Remove the one-line docs/INDEX.md (its directory pointers are covered
by docs/README.md's "when to write which document" table). Add
docs/prds/README.md documenting the PRD naming, Status lifecycle, and
section format. Repoint the AGENTS.md repository-layout list at the
new READMEs and add the decisions/ dir.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/INDEX.md

@@ -1 +0,0 @@
-Research notes live in `research/`. Product requirement docs live in `prds/`. Decision records (ADR-lite) live in `decisions/`.