feat(work): pnpm work prd-ship + auto-flip integration in sandcastle

Closes the PRD-lifecycle gap surfaced by the user: when sandcastle
finishes an epic's last task, the seed PRD should auto-flip from
approved -> shipped. Builds the mechanism, wires it into the work
CLI + state index + reviewer prompt + docs.

scripts/work/prd-ship.mjs (new):
  - parseFrontmatter / serializeFrontmatter — minimal YAML-ish parser
    sufficient for PRD frontmatter (scalar + list shapes)
  - flipPrdStatus — pure function: takes PRD text, returns new text
    with status=shipped + shipped=<date> + optional shipping-commits.
    Refuses to flip draft, idempotent fail-soft on already-shipped,
    rejects unexpected statuses
  - deriveShippingCommits — best-effort git log of the linked epic
    folder for the --auto-commits flag
  - findPrdPath — id -> path lookup under docs/work/prds/
  - runCli — wiring for `pnpm work prd-ship <id> [--commits|--auto-commits]`

scripts/work/prd-ship.test.mjs (new, 17 tests):
  - Frontmatter parser handles scalars + lists + missing frontmatter
  - flipPrdStatus covers all transitions + refusals + body/key preservation
  - findPrdPath + serializeFrontmatter coverage

scripts/work/state-builder.mjs:
  - Epic entries gain a `prd` field
  - New computeNeedsPrdShip surfaces epics done with PRD status not yet
    shipped: state.needs_prd_ship[] with action commands

scripts/work/cli.mjs:
  - New subcommand `pnpm work prd-ship <id>`

.sandcastle/reviewer.prompt.md:
  - "Epic close-out: PRD status flip" section instructing reviewer to
    check _state.json.needs_prd_ship and run the suggested action
  - JSON output extends with prd_shipped: "<id>" | null

docs/work/README.md:
  - "PRD lifecycle" section documenting the 4 statuses + auto-flip

Future PRDs follow the lifecycle automatically: decomposer refuses
draft, human flips to approved, sandcastle ships the epic, reviewer
runs prd-ship on the final task, PRD lands as shipped with its
commit trail.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/work/_state.json

@@ -1,9 +1,10 @@
 {
-  "updated_at": "2026-05-13T14:47:21.408Z",
+  "updated_at": "2026-05-13T14:51:52.854Z",
   "epics": {
     "template-reset-v1": {
       "status": "done",
       "title": "Template reset — strip setup-process noise + archive history",
+      "prd": null,
       "stories": {
         "01-template-reset": {
           "status": "done",
@@ -17,5 +18,6 @@
     }
   },
   "ready": [],
-  "blocked": []
+  "blocked": [],
+  "needs_prd_ship": []
 }