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>

parse.go

@@ -97,12 +97,23 @@ func (r *Registry) expand(spec string, visiting []string) ([]element, error) {
 			continue
 		}
 
-		// Bare token: must be a registered alias.
+		// Bare token: a registered alias, or a dynamic resolver hit.
 		r.mu.RLock()
 		target, isAlias := r.aliases[raw]
 		_, isProvider := r.providers[raw]
+		resolvers := slices.Clone(r.resolvers)
 		r.mu.RUnlock()
 
+		if !isAlias {
+			// Resolvers run without the lock — they may call back into the
+			// registry (and DB-backed ones block on I/O).
+			for _, res := range resolvers {
+				if spec, ok := res.Resolve(raw); ok && spec != "" {
+					target, isAlias = spec, true
+					break
+				}
+			}
+		}
 		if !isAlias {
 			if isProvider {
 				return nil, fmt.Errorf("%q is a provider, not an alias — use %q", raw, raw+"/<model-id>")