feat: add ToolHook interface for before/after execution interception

Introduce ToolHook with BeforeExecute/AfterExecute lifecycle methods in
ToolRegistry. Hooks enable policy enforcement and loop detection
pipelines without modifying individual tools. BeforeExecute can block
execution; AfterExecute always runs for observability.

pkg/tools/hooks.go

@@ -0,0 +1,17 @@
+package tools
+
+import "context"
+
+// ToolHook allows intercepting tool execution for policy enforcement,
+// loop detection, logging, or other cross-cutting concerns.
+//
+// Hooks are called by the ToolRegistry around tool execution:
+//   - BeforeExecute: called before the tool runs. Return non-nil error to block execution.
+//   - AfterExecute: called after the tool completes (even on error). Cannot block.
+//
+// Multiple hooks are executed in registration order. If any BeforeExecute returns
+// an error, subsequent hooks and the tool itself are skipped.
+type ToolHook interface {
+	BeforeExecute(ctx context.Context, toolName string, args map[string]interface{}) error
+	AfterExecute(ctx context.Context, toolName string, args map[string]interface{}, result *ToolResult)
+}

pkg/tools/registry.go

@@ -12,6 +12,7 @@ import (
 
 type ToolRegistry struct {
 	tools map[string]Tool
+	hooks []ToolHook
 	mu    sync.RWMutex
 }
 
@@ -21,6 +22,14 @@ func NewToolRegistry() *ToolRegistry {
 	}
 }
 
+// AddHook registers a hook that will be called around tool executions.
+// Hooks are called in registration order.
+func (r *ToolRegistry) AddHook(hook ToolHook) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	r.hooks = append(r.hooks, hook)
+}
+
 func (r *ToolRegistry) Register(tool Tool) {
 	r.mu.Lock()
 	defer r.mu.Unlock()
@@ -71,6 +80,27 @@ func (r *ToolRegistry) ExecuteWithContext(ctx context.Context, name string, args
 			})
 	}
 
+	// Run BeforeExecute hooks — any hook returning error blocks execution.
+	r.mu.RLock()
+	hooks := r.hooks
+	r.mu.RUnlock()
+
+	for _, hook := range hooks {
+		if err := hook.BeforeExecute(ctx, name, args); err != nil {
+			logger.WarnCF("tool", "Hook blocked tool execution",
+				map[string]interface{}{
+					"tool":  name,
+					"error": err.Error(),
+				})
+			blocked := ErrorResult(fmt.Sprintf("tool %q blocked by hook: %s", name, err.Error()))
+			// Still run AfterExecute hooks for observability
+			for _, h := range hooks {
+				h.AfterExecute(ctx, name, args, blocked)
+			}
+			return blocked
+		}
+	}
+
 	start := time.Now()
 	result := tool.Execute(ctx, args)
 	duration := time.Since(start)
@@ -98,6 +128,11 @@ func (r *ToolRegistry) ExecuteWithContext(ctx context.Context, name string, args
 			})
 	}
 
+	// Run AfterExecute hooks for observability
+	for _, hook := range hooks {
+		hook.AfterExecute(ctx, name, args, result)
+	}
+
 	return result
 }