Plugins can register custom agents at runtime

What

• Plugins gain a new function hook, register_agent (alongside the existing hooks for registering commands and tools), letting a plugin define a custom agent.
• The plugin SDK exposes an agent.register call that takes a name, description, and prompt (plus other fields), validating that the name only contains letters, digits, underscores, or hyphens and is no longer than 64 characters.
• Agents registered this way at runtime are merged into the session's list of available agents, tagged as coming from a plugin (source: "plugin", registeredAtRunTime: true).
• The session/engine layer gains a refreshAgents method, mirroring the existing refreshCommands, to re-read custom agent definitions on demand.

Why This lets plugins extend Claude Code with their own specialized agents without requiring a restart, the same way plugins can already register custom commands and tools.