feat(persona): add role-based review personas

Add persona system for specialized review roles. Each persona defines:
- A specific review focus (security, architecture, documentation)
- Custom system prompt additions
- Personality/tone adjustments

Built-in personas: security, architect, docs
Custom personas: load from JSON via persona-file flag

Includes workspace validation to prevent path traversal attacks.

Closes #51

review/formatter_test.go

@@ -159,3 +159,58 @@ func TestFormatMarkdown_RoleTitle(t *testing.T) {
 		t.Error("should not contain role title header when reviewer name is empty")
 	}
 }
+
+func TestFormatMarkdownWithDisplay(t *testing.T) {
+	result := &ReviewResult{
+		Verdict:        "APPROVE",
+		Summary:        "Test summary",
+		Findings:       nil,
+		Recommendation: "Test recommendation",
+	}
+
+	t.Run("with display name", func(t *testing.T) {
+		body := FormatMarkdownWithDisplay(result, "Security Specialist", "security")
+
+		// Header should use display name
+		if !strings.Contains(body, "# Security Specialist Review") {
+			t.Error("header should use display name")
+		}
+
+		// Sentinel should use sentinel name
+		if !strings.Contains(body, "<!-- review-bot:security -->") {
+			t.Error("sentinel should use sentinel name")
+		}
+
+		// Footer "Review by" should use display name
+		if !strings.Contains(body, "*Review by Security Specialist*") {
+			t.Error("footer should use display name")
+		}
+	})
+
+	t.Run("without display name", func(t *testing.T) {
+		body := FormatMarkdownWithDisplay(result, "", "reviewer")
+
+		// Should fall back to sentinel name for header
+		if !strings.Contains(body, "# Reviewer Review") {
+			t.Error("header should fall back to sentinel name")
+		}
+
+		if !strings.Contains(body, "<!-- review-bot:reviewer -->") {
+			t.Error("sentinel should use sentinel name")
+		}
+	})
+
+	t.Run("empty both names", func(t *testing.T) {
+		body := FormatMarkdownWithDisplay(result, "", "")
+
+		// Should not have header
+		if strings.Contains(body, "# ") && strings.Contains(body, " Review") {
+			t.Error("should not have header when both names empty")
+		}
+
+		// Should not have sentinel
+		if strings.Contains(body, "<!-- review-bot:") {
+			t.Error("should not have sentinel when sentinel name empty")
+		}
+	})
+}