Update defaultPrompt spec to array

[gverma-openai]

• Update plugin-creator skill spec so interface.defaultPrompt is an array of up to 3 strings
• Document truncation rules and lower per-entry max length to 128 chars
• Refresh example plugin.json and scaffolded defaults with 3 short starter prompts
• Normalize repo plugin.json prompts by shortening text and stripping baked-in @ mentions

[sayan-oai]

looks fine, but we should wait til [] parsing lands in app-server + is more broadly available so as not to break current usage.

[gverma-openai]

looks fine, but we should wait til [] parsing lands in app-server + is more broadly available so as not to break current usage.

+1. Let's also make sure malformed plugins don't break app-server and that bad specs are gracefully handled with warning errors emitted and no startup impact.