feat: conversion-driven extensions — resolvers, DefineTool, hooks, ops controls

Phase 9a (ADR-0014): Registry.RegisterResolver for dynamic tiers;
DefineTool[Args] typed tools; Usage cache/reasoning detail fields wired
through anthropic/openai/google; WithPromptCaching (Anthropic
cache_control); agent supervision hooks (WithMaxStepsFunc, WithSteer,
WithCompactor, WithToolErrorLimits + ErrToolLoop); health
Bench/Unbench/Snapshot; ChainConfig.Observer failover events.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

llm/tool.go

@@ -22,6 +22,40 @@ type Tool struct {
 	Handler func(ctx context.Context, args json.RawMessage) (any, error)
 }
 
+// DefineTool builds a typed tool: the parameter schema is derived from
+// Args (see SchemaFor) and the raw JSON arguments are unmarshaled into an
+// Args value before the handler runs.
+//
+//	weather := llm.DefineTool("get_weather", "Current weather for a city",
+//	    func(ctx context.Context, args struct {
+//	        City string `json:"city" description:"city name"`
+//	    }) (any, error) {
+//	        return lookup(args.City)
+//	    })
+//
+// Schema derivation failures panic: tools are defined at startup and an
+// unschematizable Args type is a programming error worth failing loudly on.
+func DefineTool[Args any](name, description string, fn func(ctx context.Context, args Args) (any, error)) Tool {
+	schema, err := SchemaFor[Args]()
+	if err != nil {
+		panic(fmt.Sprintf("llm: DefineTool(%q): %v", name, err))
+	}
+	return Tool{
+		Name:        name,
+		Description: description,
+		Parameters:  schema,
+		Handler: func(ctx context.Context, raw json.RawMessage) (any, error) {
+			var args Args
+			if len(raw) > 0 {
+				if err := json.Unmarshal(raw, &args); err != nil {
+					return nil, fmt.Errorf("invalid arguments for %s: %w", name, err)
+				}
+			}
+			return fn(ctx, args)
+		},
+	}
+}
+
 // ToolCall is a model's request to invoke a tool.
 type ToolCall struct {
 	// ID is the provider-assigned call id; majordomo synthesizes one for