feat(memory): add YAML frontmatter parser for JSONL sessions

- Add ParseFrontmatter function to extract YAML frontmatter from content
- Add test coverage for valid YAML, no frontmatter, and invalid YAML cases
- Use gopkg.in/yaml.v3 for parsing

pkg/memory/frontmatter.go

@@ -0,0 +1,29 @@
+package memory
+
+import (
+	"strings"
+	"gopkg.in/yaml.v3"
+)
+
+// ParseFrontmatter extracts YAML frontmatter delimited by --- from content
+// Returns: frontmatter map, remaining body, error
+func ParseFrontmatter(content string) (map[string]interface{}, string, error) {
+	result := make(map[string]interface{})
+	body := content
+
+	// Check for frontmatter delimiters
+	parts := strings.SplitN(content, "---", 3)
+	if len(parts) < 3 {
+		// No frontmatter, return empty map and original content
+		return result, body, nil
+	}
+
+	// Parse YAML
+	fmText := parts[1]
+	if err := yaml.Unmarshal([]byte(fmText), &result); err != nil {
+		return nil, "", err
+	}
+
+	body = parts[2]
+	return result, body, nil
+}

pkg/memory/frontmatter_test.go

@@ -0,0 +1,53 @@
+package memory
+
+import (
+	"testing"
+	"strings"
+)
+
+func TestParseFrontmatter_ValidYAML(t *testing.T) {
+	input := `---
+session_id: "abc123"
+timestamp: "2026-05-06T14:30:00Z"
+tags: ["coding", "bug-fix"]
+model: "claude-sonnet-4"
+---
+{"role": "user", "content": "test"}
+`
+	fm, body, err := ParseFrontmatter(input)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if fm["session_id"] != "abc123" {
+		t.Errorf("expected session_id 'abc123', got %v", fm["session_id"])
+	}
+	if len(fm["tags"].([]interface{})) != 2 {
+		t.Errorf("expected 2 tags, got %v", fm["tags"])
+	}
+	if !strings.Contains(body, `"role": "user"`) {
+		t.Errorf("expected body to contain JSON, got %s", body)
+	}
+}
+
+func TestParseFrontmatter_NoFrontmatter(t *testing.T) {
+	input := `{"role": "user", "content": "test"}`
+	_, body, err := ParseFrontmatter(input)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if !strings.Contains(body, `"role": "user"`) {
+		t.Errorf("expected body unchanged, got %s", body)
+	}
+}
+
+func TestParseFrontmatter_InvalidYAML(t *testing.T) {
+	input := `---
+invalid: [unclosed
+---
+{"role": "user"}
+`
+	_, _, err := ParseFrontmatter(input)
+	if err == nil {
+		t.Error("expected error for invalid YAML")
+	}
+}