feat(mcp): add describe_tools and annotate_tool functionality

* Implement DescribeTool for listing available MCP tools with annotations.
* Add UpsertToolAnnotation and GetToolAnnotations methods for managing tool notes.
* Create tool_annotations table for storing tool usage notes.

internal/config/config.go

@@ -36,6 +36,11 @@ type MCPConfig struct {
 	Version        string        `yaml:"version"`
 	Transport      string        `yaml:"transport"`
 	SessionTimeout time.Duration `yaml:"session_timeout"`
+	// PublicURL is the externally reachable base URL of this server (e.g. https://amcs.example.com).
+	// When set, it is used to build absolute icon URLs in the MCP server identity.
+	PublicURL string `yaml:"public_url"`
+	// Instructions is set at startup from the embedded memory.md and sent to MCP clients on initialise.
+	Instructions string `yaml:"-"`
 }
 
 type AuthConfig struct {