Bug: /plugin marketplace add writes trailing comma in known_marketplaces.json, breaking subsequent runs

Description

When adding a marketplace via /plugin marketplace add, Claude Code appends a trailing comma after the last entry in ~/.claude/plugins/known_marketplaces.json. This makes the file invalid JSON, causing all subsequent /plugin marketplace add calls to fail.

Steps to Reproduce

1. Add one or more marketplaces using /plugin marketplace add <url>
2. Check ~/.claude/plugins/known_marketplaces.json — the last entry has a trailing comma
3. Run /plugin marketplace add <another-url> again

Expected Behavior

The command succeeds.

Actual Behavior

Error: Failed to load marketplace configuration: JSON Parse error: Property name must be a string literal

Root Cause (suspected)

The code that writes known_marketplaces.json appends entries with a trailing comma, producing invalid JSON like:

{
  "some-marketplace": { ... },
  "last-marketplace": { ... },   <-- trailing comma
}

Workaround

Manually remove the trailing comma from ~/.claude/plugins/known_marketplaces.json.