[FEATURE]: Plugin discovery should support subdirectories like agents and commands #6866

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Description

The plugin discovery glob pattern uses {plugin,plugins}/*.{ts,js} which only finds plugins at the top level of the plugin directory. This is inconsistent with agents and commands which use ** for recursive discovery.

Current behavior:
// config.ts line 299
const PLUGIN_GLOB = new Bun.Glob("{plugin,plugins}/*.{ts,js}") // Non-recursive

Agents/Commands behavior:
const COMMAND_GLOB = new Bun.Glob("{command,commands}//*.md") // Recursive
const AGENT_GLOB = new Bun.Glob("{agent,agents}//*.md") // Recursive