refactor(work): drop date prefixes + move _state.json into _system/

Convention shift: epic folders + PRD filenames + frontmatter id
fields are now bare slugs. The created: timestamp (Phase 2) carries
the date; folder names don't repeat it. A future <task-id>-<slug>
shape (e.g. ClickUp) lands cleanly when that integration ships.

Renames (git mv preserves history):
- docs/work/2026-05-13-binder-wrap-helper/
    -> docs/work/binder-wrap-helper/
- docs/work/2026-05-14-library-evaluation-policy/
    -> docs/work/library-evaluation-policy/
- docs/work/2026-05-14-ci-security-and-supply-chain/
    -> docs/work/ci-security-and-supply-chain/
- docs/work/prds/2026-05-13-binder-wrap-helper.prd.md
    -> docs/work/prds/binder-wrap-helper.prd.md
- docs/work/prds/2026-05-13-coverage-architecture.prd.md
    -> docs/work/prds/coverage-architecture.prd.md
- docs/work/prds/2026-05-14-library-evaluation-policy.prd.md
    -> docs/work/prds/library-evaluation-policy.prd.md
- docs/work/prds/2026-05-14-ci-security-and-supply-chain.prd.md
    -> docs/work/prds/ci-security-and-supply-chain.prd.md

Frontmatter updates inside the renamed files: epic id, epic prd,
story epic, PRD id, PRD builds-on all drop date prefixes.

System folder + state file move:
- New docs/work/_system/ holds framework-managed state.
- docs/work/_state.json -> docs/work/_system/_state.json.
- state-builder.mjs adds _system to SKIP_FOLDERS.
- cli.mjs + state-sync-guard.mjs + .husky/pre-commit point at the
  new path.

template-reset-v1 epic deleted entirely (one-off cleanup epic from
the pre-date-convention era; status was already done).

Generator-template updates (so new artifacts ship in the right
shape):
- .sandcastle/decomposer.prompt.md emits bare-slug folder names +
  ISO created: timestamp.
- .claude/skills/to-prd/SKILL.md template uses bare-slug filename +
  bare-slug id field + ISO created: timestamp.

Doc reference updates: glossary, runbook, agent-first-workflow-
and-conformance, reviewer prompt, ADR-020, ADR-022, ADR-023 all
point at the new paths/slugs.

docs/work/ci-security-and-supply-chain/08-reviewer-prompt-update/_story.md

@@ -0,0 +1,42 @@
+---
+id: 08-reviewer-prompt-update
+epic: ci-security-and-supply-chain
+title: Sandcastle reviewer prompt update
+type: technical-story
+status: done
+feature: tooling
+depends-on: [02-socket-integration, 06-codeql-and-audit-signatures]
+blocks: [09-ci-security-guide-and-docs]
+created: 2026-05-14T18:59:12+02:00
+updated: 2026-05-14T19:16:52.691Z
+---
+
+## Goal
+
+Extend `.sandcastle/reviewer.prompt.md` with a "CI security checks" section that instructs the reviewer agent to read Socket CI output and CodeQL findings via `gh run view`, and reject the slice with specific notes if any `critical` Socket finding or `error`-severity CodeQL finding is present.
+
+## Why
+
+The sandcastle reviewer is the single composable gate for agent PRs (ADR-019 constraint). Without an explicit section, an agent reviewer has no machine-readable instruction to check Socket + CodeQL outputs and may approve a slice that introduced a flagged dependency or a static-analysis error. Landing this after Stories 02 + 06 ensures the reviewer references gates that actually exist in CI.
+
+**External dependency:** library-evaluation epic story 06 (sandcastle reviewer prompt) must be complete — the reviewer prompt must exist and have the library-trace check section that this story composes with. That epic is marked done.
+
+## Done when
+
+- `.sandcastle/reviewer.prompt.md` has a "CI security checks" section added after the existing library-trace check section.
+- The section instructs the reviewer to: (a) run `gh run view <run-id> --log` (or equivalent) for the PR's check suite; (b) scan the output for Socket findings of severity `critical` — if found, reject with notes naming the finding and referencing the failure-mode hierarchy in `docs/guides/ci-security.md`; (c) scan the output for CodeQL findings of severity `error` — same rejection pattern.
+- The reviewer composes these checks with the existing library-trace presence check (both must pass for approval).
+- `pnpm typecheck && pnpm lint && pnpm test && pnpm conformance && pnpm fallow:audit && pnpm coverage:diff` all pass (prose-only change; no executable code).
+
+## In scope
+
+- `.sandcastle/reviewer.prompt.md` — new "CI security checks" section only; existing sections unchanged.
+
+## Out of scope
+
+- Automated tests for the reviewer prompt — it's a prose runbook for an agent; success is verified manually (PRD testing decisions).
+- Extending the reviewer for `pnpm audit signatures` step failures — those surface as standard CI job failures, already handled by the reviewer's existing "all CI checks must pass" instruction.
+
+## Tasks
+
+- [x] Extend `.sandcastle/reviewer.prompt.md` with a "CI security checks" section after the library-trace check: instruct the reviewer to read `gh run view` output for Socket `critical` findings and CodeQL `error` findings, reject on either with notes naming the finding and citing `docs/guides/ci-security.md` failure-mode hierarchy; one commit, all gates pass.