docs: ri-4-001 scratchpad for PRD authority card (#1275)

scratchpads/ri-4-001-prd-authority.md

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+# Scratchpad — RI-4-001 One transitional PRD authority (RI-N3, #1275)
+
+- Objective: single PrdService authority in `@mosaicstack/prdy`; `mosaic prdy` and
+  `mission --plan` become thin adapters; mission↔PRD linkage persisted on disk;
+  Markdown export is a labeled generated view (never read back); import is
+  validated/conflict-aware with typed refusals.
+- Budget: ~35K tokens (card cap). Baselines: prdy build/lint rc=0, 0 tests;
+  mosaic build rc=0 (after root turbo build), lint rc=0, 1548 tests pass;
+  root build rc=0.
+- Plan: (1) extend store schema (version, missions linkage) (2) PrdService +
+  typed errors (3) wizard/cli route through service (4) mosaic adapters
+  (5) contract specs both packages (6) gates (7) sabotage control (8) report
+  to /var/tmp/ri-050/ri-4-001-report.md.
+- Decisions:
+  - Linkage lives ON the PRD document (`missions` array) — one authority file,
+    survives restart, no sidecar sync problems.
+  - `version` = content revision of sections/status (bumped by update/import
+    accept). Linkage writes bump `updatedAt` only, so ids/versions stay stable
+    for the card's "stable ids/versions" contract.
+  - Mission version marker = `mission.updatedAt` (gateway MissionInfo has no
+    numeric version field).
+  - Import reads YAML documents only — never the exported Markdown (keeps the
+    "no code path reads exported Markdown" invariant).
+  - Import of an existing id with identical core content → `identical` no-op;
+    divergent → typed `PrdImportConflictError` carrying proposed successor
+    (existing.version + 1, status draft, linkages preserved). Original bytes
+    untouched until explicit `acceptSuccessor`.
+  - `requirementIds` default `[]` at the mission command (no requirement
+    selection UI yet) — service accepts ids when a caller has them.
+- Progress log:
+  - [16:35] baselines captured (see above)
+  - [in progress] implementation