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/prds/binder-wrap-helper.prd.md

@@ -1,12 +1,12 @@
 ---
-id: 2026-05-13-binder-wrap-helper
+id: binder-wrap-helper
 title: Collapse binder duplication via wireUseCase helper
 type: prd
 status: approved
 author: danijel
 elicitation-session: improve-codebase-architecture-2026-05-13
 created: 2026-05-13T00:00:00Z
-updated: 2026-05-14T19:10:35.370Z
+updated: 2026-05-14T19:16:52.691Z
 ---
 
 ## Problem

docs/work/prds/ci-security-and-supply-chain.prd.md

@@ -1,13 +1,13 @@
 ---
-id: 2026-05-14-ci-security-and-supply-chain
+id: ci-security-and-supply-chain
 title: CI security + supply-chain enforcement stack
 type: prd
 status: approved
 author: danijel
 created: 2026-05-14T00:00:00Z
-updated: 2026-05-14T19:10:35.370Z
+updated: 2026-05-14T19:16:52.691Z
 adr: adr-023
-builds-on: 2026-05-14-library-evaluation-policy
+builds-on: library-evaluation-policy
 ---
 
 ## Problem
@@ -479,7 +479,7 @@ mode) — they get standard vitest coverage.
 - **Builds on ADR-022** — Library evaluation policy. ADR-022 stays
   unedited; ADR-023 §6 amends it with major-bump trigger,
   `last-revalidated` field, and Socket as the 9th hard filter.
-- **Builds on PRD** `2026-05-14-library-evaluation-policy` — many of
+- **Builds on PRD** `library-evaluation-policy` — many of
   this PRD's modules extend artifacts being built by that PRD's epic.
   The sequencing constraint in §Implementation Decisions is
   load-bearing.

docs/work/prds/coverage-architecture.prd.md

@@ -1,12 +1,12 @@
 ---
-id: 2026-05-13-coverage-architecture
+id: coverage-architecture
 title: Agent-first coverage architecture (4 layers + manifest-driven thresholds)
 type: prd
 status: shipped
 author: danijel
 elicitation-session: brainstorm-2026-05-13
 created: 2026-05-13T00:00:00Z
-updated: 2026-05-14T19:10:35.370Z
+updated: 2026-05-14T19:16:52.691Z
 shipped: 2026-05-13
 shipping-commits:
   - 7eb783a (PRD)

docs/work/prds/library-evaluation-policy.prd.md

@@ -1,11 +1,11 @@
 ---
-id: 2026-05-14-library-evaluation-policy
+id: library-evaluation-policy
 title: Library evaluation policy — skill, traces, enforcement stack
 type: prd
 status: approved
 author: danijel
 created: 2026-05-14T00:00:00Z
-updated: 2026-05-14T19:10:35.370Z
+updated: 2026-05-14T19:16:52.691Z
 adr: adr-022
 ---