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.

.claude/skills/handoff/SKILL.md

@@ -17,7 +17,7 @@ Suggest the skills the next session should use, if any. In this repo, the common
 
 Reference these artifacts by path or URL rather than inlining their content:
 
-- PRDs (`docs/work/prds/*.prd.md`), epics (`docs/work/<epic>/_epic.md`), stories (`_story.md`), tasks (`*.task.md`)
+- PRDs (`docs/work/prds/*.prd.md`), epics (`docs/work/epics/<epic>/_epic.md`), stories (`_story.md`), tasks (`*.task.md`)
 - ADRs (`docs/decisions/adr-NNN-*.md`)
 - AGENTS.md and CLAUDE.md (the next agent loads these automatically)
 - `_state.json` (orchestrator-derived; the next agent regenerates it from markdown via `pnpm work rebuild-state`)