feat(agent-plugins): read Agent Plugins packages as plugins

[Fluzko]

What does this PR do?

Part of #277.
Needed by #354.

A folder holding an Agent Plugins package (plugin.json plus skills/) is not understood where a plugin can sit: in a plugin source each skill becomes an unrelated dormant plugin, in a workspace member it is ignored, and in a dependency its skills load with no identity or validation.

Now such a package loads as one plugin with its name, version and description: dormant in a plugin source until cargo agents use (unless it declares a gate under extensions.dev.symposium), active in a workspace member, offered for consent in a dependency. Only the direct children of skills/ are skills, and a problem rejects only the smallest part it affects. Every plugin manifest can now set version and description. plugin validate, search and status label these packages. New reference page: md/reference/agent-plugins.md.

Take into account:

• Links inside a package's skill files are checked by fix(agent-plugins): keep package skill links inside the package #354.
• MCP servers in mcp.json are ignored.
• Older binaries reject version/description in Symposium.toml, so registries should adopt them after a release.

Disclosure questions

AI disclosure.

• The AI tool authored large parts of the code