docs: add plugin.json reference, cross-tool compatibility, and troubleshooting for agent plugins

[digitarald]

Addresses several documentation gaps surfaced by community feedback around authoring and troubleshooting agent plugins across VS Code, Copilot CLI, and Claude Code.

Changes

agent-plugins.md

• Plugin metadata (plugin.json) — New section documenting the required name field (kebab-case, lowercase letters/numbers/hyphens only, max 64 chars), optional fields (version, description, author, component paths), and an example plugin.json. Links to the full CLI plugin reference.
• Cross-tool compatibility — New section explaining that VS Code, Copilot CLI, and Claude Code share the plugin format, the plugin.json detection order, and key differences (hook file location, CLAUDE_PLUGIN_ROOT token, skill naming).
• Troubleshooting — New section covering: plugin not appearing, skills not loading (namespace prefix gotcha), version not updating, and cache conflicts on install with platform-specific paths.

agent-skills.md

• Tightened name field docs — Explicitly lists allowed characters and prohibits slashes, colons, dots, and namespace prefixes. Notes that invalid names cause silent load failures.
• Namespace prefix warning — Added alert explaining that plugins auto-prefix skill commands (/my-plugin:test-runner), so manual namespace prefixes are unnecessary and will break loading.

[ntrogh]

Thanks @digitarald