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/`.

docs/prds/README.md

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+# Product requirement docs
+
+One PRD per feature: what to build, why, and how it's scoped. The PRD
+is the durable spec — it should stand on its own without a forge issue
+thread (see [`../README.md`](../README.md) for when a PRD is the right
+document vs. a research note or a decision record).
+
+## Naming and numbering
+
+`NNNN-kebab-title.md`, zero-padded and sequential (`0024-…`, `0025-…`).
+Numbers are never reused; gaps are fine (there is no 0005). The number
+is assigned at creation and stays fixed for the life of the doc.
+
+## Status
+
+The `Status:` line near the top tracks the PRD's lifecycle:
+
+- **Draft** — proposed, not yet shipped.
+- **Active** — the design has shipped to `main` and is in effect.
+- **Superseded by [PRD NNNN](…)** — replaced by a later PRD; kept for history.
+- **Retargeted by [PRD NNNN](…)** — folded into a later PRD's scope.
+
+## Format
+
+```markdown
+# PRD NNNN: <short title>
+
+- **Status:** Draft
+- **Author:** <who>
+- **Created:** YYYY-MM-DD
+- **Issue:** #<n>            # optional — convenience pointer only
+
+## Summary
+One paragraph: what this builds and the pain it solves.
+
+## Problem
+The current state and why it's inadequate.
+
+## Goals / Success Criteria
+Bullets a reviewer can check the finished work against.
+
+## Non-goals
+What this explicitly does not do — and won't, to head off scope creep.
+
+## Scope
+In scope / out of scope, when the boundary needs spelling out.
+
+## Design
+How it works: schema, data flow, diagrams, algorithms as needed.
+
+## Implementation chunks
+Ordered, mergeable steps (optional; for multi-PR features).
+
+## Open questions
+Unresolved decisions — resolve or fold into Design before shipping.
+```
+
+Sections are a guide, not a straitjacket: drop the ones a given PRD
+doesn't need (a small change rarely needs Scope or Implementation
+chunks) and add others where they help (e.g. Testing strategy,
+Alternatives considered, References). Keep the rationale self-contained
+— inline the reasoning rather than linking out to an issue thread, so
+the PRD survives a forge migration.