Plugin trait and registry for the Astrid secure agent runtime SDK.

Provides the core abstractions for extending Astrid with plugins:

• [PluginId]: Stable, human-readable plugin identifier
• [PluginManifest]: Describes a plugin's identity, entry point, and capabilities
• [Plugin]: Trait for plugin lifecycle (load/unload) and tool provision
• [PluginTool]: Trait for tools provided by plugins (mirrors BuiltinTool)
• [PluginContext] / [PluginToolContext]: Execution contexts with scoped KV storage
• [PluginRegistry]: Registry for loaded plugins with cross-plugin tool lookup
• [discover_manifests]: Filesystem discovery of plugin.toml manifests

Tool Naming Convention

Plugin tools are exposed to the LLM as plugin:{plugin_id}:{tool_name}, which avoids collision with built-in tools (no colons) and MCP tools (server:tool — single colon).

Storage Isolation

Each plugin gets a ScopedKvStore pre-bound to the namespace plugin:{plugin_id}. Plugins cannot access each other's data.