Plugin install command produces no output on success

Description

When installing a plugin via /plugin install, a successful installation produces no output. Claude Code displays "(no content)" since the command returned empty stdout.

Steps to Reproduce

1. Add a marketplace: /plugin marketplace add AhmedHamadto/software-forge
2. Install a plugin: /plugin install software-forge@AhmedHamadto/software-forge
3. Observe: no output — Claude Code shows "(no content)"
4. Run the same command again: now it says Plugin 'software-forge@software-forge' is already installed.

Expected Behavior

A success confirmation message, e.g.:

Successfully installed software-forge@software-forge v1.1.0

Actual Behavior

No output at all. The user has no way to know the install succeeded without running the command again or checking installed_plugins.json manually.

Additional Context

• The "already installed" path does produce output, so the inconsistency is confusing
• Failure cases (e.g., invalid manifest) also produce output
• Only the success path is silent