fix(agent): set AgentUUID in memoryflush SystemPromptConfig

memoryflush.go constructed SystemPromptConfig without AgentUUID, while
loop_history.go set both AgentID (agent_key) and AgentUUID. buildRuntimeSection
renders below the cache boundary so the omission did not bust the prompt
cache — but the runtime identity line dropped the UUID during compaction
flush turns.

Extract a pure helper buildMemoryFlushPromptConfig so tests can assert the
config shape without a full Loop fixture (M7 mitigation from red-team
review).

Phase 1 Fix A of agent identity hardening.

internal/agent/memoryflush.go

@@ -67,6 +67,28 @@ func ResolveMemoryFlushSettings(compaction *config.CompactionConfig) *MemoryFlus
 	return settings
 }
 
+// buildMemoryFlushPromptConfig returns the SystemPromptConfig used by the
+// memory flush turn. Extracted as a pure function so tests can assert the
+// config shape (specifically, that AgentUUID is populated) without building
+// a full Loop fixture. See Phase 1 Fix A and M7 mitigation.
+func buildMemoryFlushPromptConfig(
+	agentID, agentUUID, model, workspace string,
+	toolNames []string,
+	hasMemory bool,
+	providerType string,
+) SystemPromptConfig {
+	return SystemPromptConfig{
+		AgentID:      agentID,
+		AgentUUID:    agentUUID,
+		Model:        model,
+		Workspace:    workspace,
+		Mode:         PromptMinimal,
+		ToolNames:    toolNames,
+		HasMemory:    hasMemory,
+		ProviderType: providerType,
+	}
+}
+
 // shouldRunMemoryFlush checks whether a memory flush should run before compaction.
 // Flush always runs when compaction triggers (called inside maybeSummarize),
 // gated only by enabled/memory checks and a dedup guard per compaction cycle.
@@ -108,16 +130,19 @@ func (l *Loop) runMemoryFlush(ctx context.Context, sessionKey string, settings *
 	flushPrompt := strings.ReplaceAll(settings.Prompt, "YYYY-MM-DD", today)
 	flushSystemPrompt := strings.ReplaceAll(settings.SystemPrompt, "YYYY-MM-DD", today)
 
-	// System prompt: combine agent's normal system prompt context with flush system prompt
-	systemPrompt := BuildSystemPrompt(SystemPromptConfig{
-		AgentID:      l.id,
-		Model:        l.model,
-		Workspace:    l.workspace,
-		Mode:         PromptMinimal,
-		ToolNames:    l.filteredToolNames(),
-		HasMemory:    l.hasMemory,
-		ProviderType: providerTypeOf(l.provider),
-	})
+	// System prompt: combine agent's normal system prompt context with flush system prompt.
+	// Config construction extracted to buildMemoryFlushPromptConfig for testability — the
+	// AgentUUID field here mirrors loop_history.go:199-201 and must stay in sync
+	// (regression: missing AgentUUID caused identity drift in DomainEvents — see PR #826).
+	systemPrompt := BuildSystemPrompt(buildMemoryFlushPromptConfig(
+		l.id,
+		l.agentUUID.String(),
+		l.model,
+		l.workspace,
+		l.filteredToolNames(),
+		l.hasMemory,
+		providerTypeOf(l.provider),
+	))
 	systemPrompt += "\n\n" + flushSystemPrompt
 
 	messages = append(messages, providers.Message{

internal/agent/memoryflush_test.go

@@ -0,0 +1,70 @@
+package agent
+
+import (
+	"testing"
+
+	"github.com/google/uuid"
+)
+
+// TestBuildMemoryFlushPromptConfig_AgentUUIDPopulated asserts Fix A — the
+// SystemPromptConfig returned by buildMemoryFlushPromptConfig carries the
+// AgentUUID. Historically missing; loop_history.go set it but memoryflush.go
+// did not, leading to identity drift (see PR #826 / brainstorm trap zone 2).
+func TestBuildMemoryFlushPromptConfig_AgentUUIDPopulated(t *testing.T) {
+	u := uuid.New()
+	cfg := buildMemoryFlushPromptConfig(
+		"test-agent",
+		u.String(),
+		"claude-opus",
+		"/workspace",
+		[]string{"read_file", "write_file"},
+		true,
+		"anthropic",
+	)
+
+	if cfg.AgentUUID != u.String() {
+		t.Errorf("AgentUUID = %q, want %q", cfg.AgentUUID, u.String())
+	}
+	if cfg.AgentID != "test-agent" {
+		t.Errorf("AgentID = %q, want %q", cfg.AgentID, "test-agent")
+	}
+	if cfg.Model != "claude-opus" {
+		t.Errorf("Model = %q, want %q", cfg.Model, "claude-opus")
+	}
+	if cfg.Workspace != "/workspace" {
+		t.Errorf("Workspace = %q, want %q", cfg.Workspace, "/workspace")
+	}
+	if cfg.Mode != PromptMinimal {
+		t.Errorf("Mode = %q, want %q", cfg.Mode, PromptMinimal)
+	}
+	if !cfg.HasMemory {
+		t.Error("HasMemory should be true")
+	}
+	if cfg.ProviderType != "anthropic" {
+		t.Errorf("ProviderType = %q, want %q", cfg.ProviderType, "anthropic")
+	}
+	if len(cfg.ToolNames) != 2 {
+		t.Errorf("ToolNames len = %d, want 2", len(cfg.ToolNames))
+	}
+}
+
+// TestBuildMemoryFlushPromptConfig_EmptyAgentUUID guards against a regression where
+// a Loop with uuid.Nil (zero value) would still produce a non-empty AgentUUID string.
+// The .String() of uuid.Nil is "00000000-0000-0000-0000-000000000000" — not empty,
+// but it MUST be identifiable downstream so the publish-time observer in eventbus
+// can still warn on it after Fix C wires validateAgentID into bus.Publish.
+func TestBuildMemoryFlushPromptConfig_ZeroUUIDStringified(t *testing.T) {
+	cfg := buildMemoryFlushPromptConfig(
+		"zero-agent",
+		uuid.Nil.String(),
+		"m",
+		"/w",
+		nil,
+		false,
+		"p",
+	)
+
+	if cfg.AgentUUID != uuid.Nil.String() {
+		t.Errorf("AgentUUID = %q, want %q", cfg.AgentUUID, uuid.Nil.String())
+	}
+}