plugin marketplace add succeeds but doesn't register in known_marketplaces.json

Bug

/plugin marketplace add reports success and downloads marketplace files, but does not add an entry to ~/.claude/plugins/known_marketplaces.json. This causes /plugin install and /plugin marketplace list to not recognize the marketplace.

Steps to reproduce

1. Run /plugin marketplace add RhombusSystems/claude-code-plugins
2. Output: Successfully added marketplace: rhombus-plugins
3. Run /plugin marketplace list — only claude-plugins-official is shown
4. Run /plugin install rhombus-user@rhombus-plugins — Plugin "rhombus-user" not found in any marketplace

Observed

• Files are correctly downloaded to ~/.claude/plugins/marketplaces/rhombus-plugins/
• known_marketplaces.json only contains the claude-plugins-official entry — no rhombus-plugins entry was added

Expected

/plugin marketplace add should add the marketplace to known_marketplaces.json so that subsequent commands can discover it.

Workaround

Manually add the entry to ~/.claude/plugins/known_marketplaces.json:

"rhombus-plugins": {
  "source": { "source": "github", "repo": "RhombusSystems/claude-code-plugins" },
  "installLocation": "/Users/brandonsalzberg/.claude/plugins/marketplaces/rhombus-plugins",
  "lastUpdated": "2026-03-20T..."
}