fix(plugin): move marketplace.json to repo root so GitHub install works

[kimyouknow]

Summary

Follow-up to #374. The plugin is still not installable via the documented GitHub URL flow because .claude-plugin/marketplace.json lives under packages/plugin/, not at the repo root. claude plugin marketplace add always looks for the manifest at <repo-root>/.claude-plugin/marketplace.json — --sparse only limits checkout scope, not the manifest lookup path.

Reproduction (on current main)

$ claude plugin marketplace add https://github.com/toss/react-simplikit \
    --sparse packages/plugin
✘ Failed to add marketplace: Marketplace file not found at
  <tempdir>/.claude-plugin/marketplace.json

Changes

File	Action
.claude-plugin/marketplace.json (new at repo root)	Created — points source: "./packages/plugin" at the plugin
packages/plugin/.claude-plugin/marketplace.json	Deleted — moved to root
packages/plugin/README.md	Updated install command to --sparse .claude-plugin packages/plugin
packages/plugin/.claude-plugin/plugin.json	Untouched — plugin identity still lives with the plugin

Monorepo pattern

This matches the canonical monorepo marketplace layout:

<repo-root>/
├── .claude-plugin/marketplace.json      ← catalog (root)
└── packages/plugin/
    └── .claude-plugin/plugin.json       ← plugin identity (co-located)

Verified locally

Step	Command	Result
1	claude plugin validate .	✅ Passed
2	claude plugin marketplace add <repo-root> --scope local	✅ Added
3	claude plugin install react-design-philosophy@react-design-philosophy	✅ Installed v0.1.0, enabled

(Local path install mirrors the post-clone state, so this proves the real install flow will work once merged.)

Test plan

• After merge, run the README command from a fresh directory against the public repo
• Confirm plugin shows up in claude plugin list with status enabled

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