feat(openprd): revise standard to DIP-style numbered proposals (0.2) (#96)

Reshape OpenPRD from a private single-file (prd/<slug>/prd.md, gitignored) into a
published, numbered proposal collection like BIP/EIP/DIP: prd/NNNN-slug.md +
0000-template.md + a README index, committed to the repo, with a lifecycle
(Draft → Review → Accepted → Final; Rejected/Withdrawn/Superseded).

Tools (e.g. moshcode /prd) consume this to publish PRDs into whatever repo the
user is working in.

- docs/openprd.md — rewritten: numbering, lifecycle, directory layout, conformance.
- docs/openprd/0000-template.md — the canonical template.
- packages/schemas/schemas/openprd-prd.schema.json — 4-digit id, status enum,
  authors, discussion/implementation, supersedes/superseded-by.
- fixture updated to 0.2.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

docs/openprd/0000-template.md

@@ -0,0 +1,50 @@
+---
+openprd: "0.2"
+id: "0000"
+title: "Short imperative title — start with a verb if possible"
+status: Draft
+authors:
+  - you@example.com
+created: 2026-01-01
+updated: 2026-01-01
+repo:
+discussion:
+implementation:
+tags:
+supersedes:
+superseded-by:
+---
+
+## Problem
+
+The user/business problem, and why it matters now. Cite the ask, the incident,
+or the constraint — not aesthetics.
+
+## Goals
+
+What success looks like, as outcomes (not features).
+
+## Non-Goals
+
+Explicitly out of scope, to bound the work.
+
+## Users
+
+Who this is for; personas or segments.
+
+## Requirements
+
+- R1 [P0] First required capability.
+- R2 [P1] Next capability.
+
+## UX Notes
+
+Flows, states, and constraints that shape the experience.
+
+## Success Metrics
+
+How the goals will be measured.
+
+## Risks & Open Questions
+
+- Known risk or decision still owed.