docs: hoist "when to write which document" to docs/README.md

Move the document-type comparison out of docs/decisions/README.md
(where it only surfaced if you were already in the decisions dir) up
to a new docs/README.md, renamed "When to write which document".
Leave a pointer from the decisions README.

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

docs/README.md

@@ -0,0 +1,18 @@
+# Docs
+
+How this project records what it builds and why — and a guide to
+picking the right document for what you're capturing.
+
+## When to write which document
+
+| Artifact | For |
+|---|---|
+| **PRD** (`docs/prds/`) | A feature: what to build, scope, success criteria. |
+| **Research note** (`docs/research/`) | A landscape/tradeoff investigation. |
+| **Decision record** (`docs/decisions/`) | A decision that isn't itself a feature — a policy, a convention, a "we will / won't do this," or a load-bearing choice made inside a larger PRD that deserves to be discoverable on its own. |
+
+A decision that's fully specified by a PRD doesn't need duplicating in
+a decision record. Write one when the *decision* would otherwise be
+buried in prose, lost in an issue thread, or have no in-repo home at
+all (small requests that don't merit a PRD; non-feature choices like
+merge strategy or a trust posture).