feat: load personas from target repo .review-bot/personas/

- Add RepoContentFetcher interface for fetching repo files
- Add LoadRepoPersonas() to load custom personas from repo
- Add LoadPersonaWithFallback() to check repo then built-in
- Add ListAllPersonas() to merge repo + built-in persona names
- Repo personas take precedence over built-ins with same name

Closes #60

review/persona_test.go

@@ -1,6 +1,7 @@
 package review
 
 import (
+	"context"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -776,3 +777,242 @@ identity: test identity
 		t.Errorf("Name = %q, want %q", p.Name, "test")
 	}
 }
+
+// MockRepoFetcher is a mock implementation of RepoContentFetcher for testing.
+type MockRepoFetcher struct {
+	Contents map[string][]ContentEntry // path -> entries
+	Files    map[string]string         // path -> content
+}
+
+func (m *MockRepoFetcher) ListContents(ctx context.Context, owner, repo, path string) ([]ContentEntry, error) {
+	key := fmt.Sprintf("%s/%s/%s", owner, repo, path)
+	entries, ok := m.Contents[key]
+	if !ok {
+		return nil, fmt.Errorf("path not found: %s", path)
+	}
+	return entries, nil
+}
+
+func (m *MockRepoFetcher) GetFileContent(ctx context.Context, owner, repo, filepath string) (string, error) {
+	key := fmt.Sprintf("%s/%s/%s", owner, repo, filepath)
+	content, ok := m.Files[key]
+	if !ok {
+		return "", fmt.Errorf("file not found: %s", filepath)
+	}
+	return content, nil
+}
+
+func TestLoadRepoPersonas(t *testing.T) {
+	validPersona := `name: custom-security
+display_name: Custom Security
+identity: |
+  You are a custom security reviewer.
+focus:
+  - SQL injection
+  - XSS attacks
+ignore:
+  - Code style
+severity:
+  major: Critical vulnerabilities
+  minor: Potential issues
+  nit: Suggestions
+`
+
+	tests := []struct {
+		name      string
+		fetcher   *MockRepoFetcher
+		wantCount int
+		wantNames []string
+	}{
+		{
+			name: "no personas directory",
+			fetcher: &MockRepoFetcher{
+				Contents: map[string][]ContentEntry{},
+				Files:    map[string]string{},
+			},
+			wantCount: 0,
+		},
+		{
+			name: "empty personas directory",
+			fetcher: &MockRepoFetcher{
+				Contents: map[string][]ContentEntry{
+					"owner/repo/.review-bot/personas": {},
+				},
+				Files: map[string]string{},
+			},
+			wantCount: 0,
+		},
+		{
+			name: "one valid persona",
+			fetcher: &MockRepoFetcher{
+				Contents: map[string][]ContentEntry{
+					"owner/repo/.review-bot/personas": {
+						{Name: "custom-security.yaml", Path: ".review-bot/personas/custom-security.yaml", Type: "file"},
+					},
+				},
+				Files: map[string]string{
+					"owner/repo/.review-bot/personas/custom-security.yaml": validPersona,
+				},
+			},
+			wantCount: 1,
+			wantNames: []string{"custom-security"},
+		},
+		{
+			name: "skip non-yaml files",
+			fetcher: &MockRepoFetcher{
+				Contents: map[string][]ContentEntry{
+					"owner/repo/.review-bot/personas": {
+						{Name: "custom-security.yaml", Path: ".review-bot/personas/custom-security.yaml", Type: "file"},
+						{Name: "readme.md", Path: ".review-bot/personas/readme.md", Type: "file"},
+						{Name: "subdir", Path: ".review-bot/personas/subdir", Type: "dir"},
+					},
+				},
+				Files: map[string]string{
+					"owner/repo/.review-bot/personas/custom-security.yaml": validPersona,
+				},
+			},
+			wantCount: 1,
+			wantNames: []string{"custom-security"},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			ctx := context.Background()
+			personas, err := LoadRepoPersonas(ctx, tt.fetcher, "owner", "repo")
+			if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+			if len(personas) != tt.wantCount {
+				t.Errorf("got %d personas, want %d", len(personas), tt.wantCount)
+			}
+			for _, name := range tt.wantNames {
+				if _, ok := personas[name]; !ok {
+					t.Errorf("missing expected persona %q", name)
+				}
+			}
+		})
+	}
+}
+
+func TestLoadPersonaWithFallback(t *testing.T) {
+	customSecurity := `name: security
+display_name: Custom Security Override
+identity: |
+  Custom security reviewer for this repo.
+focus:
+  - Repo-specific security
+ignore:
+  - General stuff
+severity:
+  major: Critical
+  minor: Warning
+  nit: Info
+`
+
+	tests := []struct {
+		name            string
+		fetcher         *MockRepoFetcher
+		personaName     string
+		wantDisplayName string
+		wantErr         bool
+	}{
+		{
+			name: "repo persona overrides builtin",
+			fetcher: &MockRepoFetcher{
+				Contents: map[string][]ContentEntry{
+					"owner/repo/.review-bot/personas": {
+						{Name: "security.yaml", Path: ".review-bot/personas/security.yaml", Type: "file"},
+					},
+				},
+				Files: map[string]string{
+					"owner/repo/.review-bot/personas/security.yaml": customSecurity,
+				},
+			},
+			personaName:     "security",
+			wantDisplayName: "Custom Security Override",
+		},
+		{
+			name: "fallback to builtin when repo has no override",
+			fetcher: &MockRepoFetcher{
+				Contents: map[string][]ContentEntry{},
+				Files:    map[string]string{},
+			},
+			personaName:     "security",
+			wantDisplayName: "Security Specialist",
+		},
+		{
+			name: "unknown persona",
+			fetcher: &MockRepoFetcher{
+				Contents: map[string][]ContentEntry{},
+				Files:    map[string]string{},
+			},
+			personaName: "nonexistent",
+			wantErr:     true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			ctx := context.Background()
+			p, err := LoadPersonaWithFallback(ctx, tt.fetcher, "owner", "repo", tt.personaName)
+			if tt.wantErr {
+				if err == nil {
+					t.Error("expected error, got nil")
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+			if p.DisplayName != tt.wantDisplayName {
+				t.Errorf("DisplayName = %q, want %q", p.DisplayName, tt.wantDisplayName)
+			}
+		})
+	}
+}
+
+func TestListAllPersonas(t *testing.T) {
+	customPersona := `name: repo-specific
+display_name: Repo Specific
+identity: A repo-specific reviewer.
+focus: []
+ignore: []
+severity:
+  major: Major
+  minor: Minor
+  nit: Nit
+`
+
+	fetcher := &MockRepoFetcher{
+		Contents: map[string][]ContentEntry{
+			"owner/repo/.review-bot/personas": {
+				{Name: "repo-specific.yaml", Path: ".review-bot/personas/repo-specific.yaml", Type: "file"},
+			},
+		},
+		Files: map[string]string{
+			"owner/repo/.review-bot/personas/repo-specific.yaml": customPersona,
+		},
+	}
+
+	ctx := context.Background()
+	names := ListAllPersonas(ctx, fetcher, "owner", "repo")
+
+	// Should include both built-ins and repo-specific
+	builtins := ListBuiltinPersonas()
+	if len(names) <= len(builtins) {
+		t.Error("expected more personas than just built-ins")
+	}
+
+	// Check repo-specific is included
+	found := false
+	for _, name := range names {
+		if name == "repo-specific" {
+			found = true
+			break
+		}
+	}
+	if !found {
+		t.Error("repo-specific persona not found in list")
+	}
+}