朱子纯 / erp-workflow-plugin

refactor: group skills by phase — skills/plan/ + skills/coding/

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).