fix(plugin): add marketplace.json so the install command actually works

[kimyouknow]

Summary

After #366 merged, the react-design-philosophy plugin is impossible to install because it ships only plugin.json, not the marketplace.json required by claude plugin. The README install command also references CLI flags (--source, --url, --path) that don't exist in the current claude plugin CLI.

This PR fixes both.

Evidence of the bug on current main

$ claude plugin marketplace add /path/to/packages/plugin
✘ Failed to add marketplace: Marketplace file not found at
  packages/plugin/.claude-plugin/marketplace.json

$ claude plugin install --help
Usage: claude plugin install [options] <plugin>
Options:
  -s, --scope <scope>    # (no --source / --url / --path)

Changes

1. Add packages/plugin/.claude-plugin/marketplace.json — declares a single-plugin marketplace whose source points at "./" (plugin and marketplace share the same directory)
2. Rewrite README ## Install section — replace the non-existent --source git-subdir flags with the real two-step flow

Verified locally end-to-end

Step	Command	Result
1	claude plugin validate .	✅ Passed
2	claude plugin marketplace add . --scope local	✅ Added react-design-philosophy
3	claude plugin install react-design-philosophy@react-design-philosophy	✅ Installed v0.1.0, enabled
4	claude plugin list	✅ Listed
5	claude plugin uninstall …	✅ Cleanly removed

Test plan

• After merge, run the README command set against the published repo from a fresh directory
• Confirm /react-design-principles, /react-hook-review, /react-hook-writing are available after install

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Latest commit: 698c68a

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