朱子纯 / erp-workflow-plugin

- delete frontend-scope-lock skill + docs/06 templates (static + module-pagelist)
- A5 downstream-gen gains prototype/ gate + FE-list derivation -> docs/08 § 三
- plan-start: drop A6 dispatch row, gate 5->4 items; coding-start A0~A6 -> A0~A5
- Design Tokens single source = src/styles/tokens.css (code-reviewer, coding.mjs)
- plugin.json skills 9->8; docs-09 tree drops 06-UI; docs-08 drops A6 section

Visual clarity: the file tree now matches the README's phase model.
- skills/plan/   — 8 skills (plan-start entry + A0~A6)
- skills/coding/ — 1 skill (coding-start thin entry)

Slugs unchanged. /erp-workflow:<name> still resolves identically because
each SKILL.md declares its own `name:` frontmatter (verified for all 9).

Plugin loader treats the default `./skills/` scan as flat, so the nested
layout is opted into via an explicit path array in plugin.json — the only
mechanism Claude Code provides for nested skill discovery.

Changes:
- mv skills/{plan-start,project-init,scope-lock,skeleton-gen,
       db-design-gen,db-init,downstream-gen,frontend-scope-lock}
     → skills/plan/<same>/
- mv skills/coding-start → skills/coding/coding-start/
- .claude-plugin/plugin.json: skills[] now lists all 9 explicit paths
- README.md: directory tree + Skill 清单 heading updated to reflect grouping

No code touches any skill path (only README did, and only in the tree
diagram). lib tests still pass (62/62).