refactor(work): move epic folders into docs/work/epics/

The previous layout placed epic folders directly under docs/work/
alongside prds/ and _system/. Tightening: epics now live in their
own docs/work/epics/ subfolder, peer to prds/ and _system/. Same
shape as the existing prds/ bucket.

Final docs/work/ layout:
  README.md
  prds/<slug>.prd.md
  _system/_state.json
  epics/<slug>/_epic.md + <story-folder>/_story.md

Renames (git mv preserves history):
- docs/work/binder-wrap-helper/
    -> docs/work/epics/binder-wrap-helper/
- docs/work/library-evaluation-policy/
    -> docs/work/epics/library-evaluation-policy/
- docs/work/ci-security-and-supply-chain/
    -> docs/work/epics/ci-security-and-supply-chain/

Tooling updates:
- state-builder.mjs walks workRoot/epics/ directly; SKIP_FOLDERS
  obsoleted (no more sibling folders to filter out).
- dispatch.mjs's findNextTask, tickStoryBulletInEpic, and
  flipEpicDoneIfAllStoriesDone all join with "epics" segment.
- prd-ship.mjs's deriveShippingCommits walks workRoot/epics/ and
  git-logs docs/work/epics/<epic>/.
- decomposer.prompt.md emits epics under docs/work/epics/<epic-id>/.
- handoff + grill-with-docs glossary references updated.
- Glossary entry for Epic updated.

Reserved future shape: when a task-tracker integration (ClickUp,
Linear) ships, the epics/ subfolder hosts <task-id>-<slug>/
folders. Today it just hosts bare slugs.

docs/glossary.md

@@ -273,7 +273,7 @@ The top-level requirements doc at `docs/work/prds/<date>-<slug>.prd.md`. Status
 _Avoid:_ spec (in this template the **ADR** is the durable design record; PRDs are the implementation seed).
 
 **Epic**:
-A large body of work, one folder under `docs/work/<epic-slug>/` with `_epic.md`. Decomposed from a PRD.
+A large body of work, one folder under `docs/work/epics/<epic-slug>/` with `_epic.md`. Decomposed from a PRD.
 
 **Story**:
 One use case or one technical capability, one folder under `<epic>/<story-slug>/` with `_story.md`. Type metadata: `user-story` or `technical-story`.