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>

generate.go

@@ -14,6 +14,12 @@ import (
 // suitable for WithSchema and tool parameters.
 func SchemaFor[T any]() (json.RawMessage, error) { return llm.SchemaFor[T]() }
 
+// DefineTool re-exports llm.DefineTool: a typed tool whose parameter schema
+// is derived from Args and whose handler receives decoded arguments.
+func DefineTool[Args any](name, description string, fn func(ctx context.Context, args Args) (any, error)) Tool {
+	return llm.DefineTool(name, description, fn)
+}
+
 // Generate performs a structured-output request and unmarshals the result
 // into T: the schema is derived from T (llm.SchemaFor), injected via the
 // provider's native structured-output mechanism, and the response text is