feat(agent): wire AGENTS.md discovery into the agent loop's system prompt

Export persona.DiscoverAgentsMD and add agent.Config.AgentsMD so project
and global AGENTS.md rules actually reach the model. Previously
buildInitialMessages always called AssembleSystemPrompt with an empty
string, so no AGENTS.md content was ever injected despite the discovery
logic already existing.

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

pkg/agent/loop.go

@@ -42,6 +42,7 @@ type Config struct {
 	OnIteration        OnIterationHook
 	ToolTimeout        time.Duration
 	ChatTemplateKwargs map[string]any // passed to the LLM provider (e.g. Qwen enable_thinking)
+	AgentsMD           string         // discovered AGENTS.md content, folded into the system prompt
 }
 
 // Iteration represents a single cycle of the agent loop.
@@ -230,7 +231,7 @@ func (l *Loop) RunStream(ctx context.Context, input string, history ...llm.Messa
 }
 
 func (l *Loop) buildInitialMessages(input string, history []llm.Message) []llm.Message {
-	systemPrompt := persona.AssembleSystemPrompt(l.cfg.Persona, "")
+	systemPrompt := persona.AssembleSystemPrompt(l.cfg.Persona, l.cfg.AgentsMD)
 	messages := make([]llm.Message, 0, len(history)+2)
 	messages = append(messages, llm.Message{Role: llm.RoleSystem, Content: systemPrompt})
 	messages = append(messages, history...)

pkg/agent/loop_test.go

@@ -6,6 +6,7 @@ import (
 	"errors"
 	"fmt"
 	"iter"
+	"strings"
 	"testing"
 	"time"
 
@@ -116,6 +117,30 @@ func TestRun_NoToolCalls(t *testing.T) {
 	}
 }
 
+func TestRun_IncludesAgentsMD(t *testing.T) {
+	var capturedSystemPrompt string
+	mockClient := &mockLLM{
+		generateFunc: func(ctx context.Context, req llm.CompletionRequest) (llm.CompletionResponse, error) {
+			capturedSystemPrompt = req.Messages[0].Content
+			return llm.CompletionResponse{Content: "done"}, nil
+		},
+	}
+
+	loop := New(Config{
+		LLM:      mockClient,
+		Persona:  persona.DefaultPersona(),
+		Tools:    tools.NewRegistry(),
+		AgentsMD: "Never edit go.mod directly.",
+	})
+
+	if _, err := loop.Run(context.Background(), "Hello"); err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if !strings.Contains(capturedSystemPrompt, "Never edit go.mod directly.") {
+		t.Errorf("expected system prompt to include AGENTS.md content, got %q", capturedSystemPrompt)
+	}
+}
+
 func TestRun_ToolCalls(t *testing.T) {
 	registry := tools.NewRegistry()
 	registry.Register(tools.Tool{

pkg/persona/persona.go

@@ -79,8 +79,8 @@ func AssembleSystemPrompt(p Persona, agentsMD string) string {
 	return strings.Join(parts, "\n\n")
 }
 
-// discoverAgentsMD walks up the directory tree looking for AGENTS.md files.
-func discoverAgentsMD(root string) string {
+// DiscoverAgentsMD walks up the directory tree looking for AGENTS.md files.
+func DiscoverAgentsMD(root string) string {
 	var parts []string
 	current := root
 

pkg/persona/persona_test.go

@@ -45,9 +45,9 @@ func TestDiscoverAgentsMD(t *testing.T) {
 	agentsPath := filepath.Join(tmpDir, "AGENTS.md")
 	os.WriteFile(agentsPath, []byte("test instructions"), 0644)
 
-	result := discoverAgentsMD(tmpDir)
+	result := DiscoverAgentsMD(tmpDir)
 	if !contains(result, "test instructions") {
-		t.Error("expected discoverAgentsMD to find AGENTS.md")
+		t.Error("expected DiscoverAgentsMD to find AGENTS.md")
 	}
 }