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.go

@@ -2,10 +2,12 @@ package review
 
 import (
 	"bytes"
+	"context"
 	"embed"
 	"encoding/json"
 	"fmt"
 	"os"
+	"path/filepath"
 	"sort"
 	"strings"
 	"unicode/utf8"
@@ -28,6 +30,9 @@ const MaxYAMLDepth = 20
 // This prevents DoS via wide-but-shallow structures that bypass depth limits.
 const MaxYAMLNodes = 1000
 
+// RepoPersonasPath is the path within a repository where custom personas are stored.
+const RepoPersonasPath = ".review-bot/personas"
+
 // Persona defines a specialized review role with focused expertise.
 type Persona struct {
 	Name         string   `json:"name" yaml:"name"`
@@ -48,6 +53,20 @@ type Severity struct {
 	Nit   string `json:"nit" yaml:"nit"`
 }
 
+// RepoContentFetcher is an interface for fetching file content from a repository.
+// This allows LoadRepoPersonas to work with any client that can list and fetch files.
+type RepoContentFetcher interface {
+	ListContents(ctx context.Context, owner, repo, path string) ([]ContentEntry, error)
+	GetFileContent(ctx context.Context, owner, repo, filepath string) (string, error)
+}
+
+// ContentEntry represents a file or directory entry (mirrors gitea.ContentEntry).
+type ContentEntry struct {
+	Name string `json:"name"`
+	Path string `json:"path"`
+	Type string `json:"type"` // "file" or "dir"
+}
+
 // LoadPersona loads a persona from a JSON or YAML file path.
 // Format is detected by file extension: .yaml/.yml for YAML, .json or other for JSON.
 // Files larger than MaxPersonaFileSize are rejected.
@@ -130,6 +149,92 @@ func ListBuiltinPersonas() []string {
 	return names
 }
 
+// LoadRepoPersonas loads custom personas from a repository's .review-bot/personas/ directory.
+// Returns an empty map if the directory doesn't exist or is empty.
+// Repo personas take precedence over built-in personas with the same name.
+func LoadRepoPersonas(ctx context.Context, client RepoContentFetcher, owner, repo string) (map[string]*Persona, error) {
+	personas := make(map[string]*Persona)
+
+	entries, err := client.ListContents(ctx, owner, repo, RepoPersonasPath)
+	if err != nil {
+		// Directory doesn't exist — not an error, just no custom personas
+		return personas, nil
+	}
+
+	for _, entry := range entries {
+		if entry.Type != "file" {
+			continue
+		}
+		// Only load YAML files
+		ext := strings.ToLower(filepath.Ext(entry.Name))
+		if ext != ".yaml" && ext != ".yml" {
+			continue
+		}
+
+		content, err := client.GetFileContent(ctx, owner, repo, entry.Path)
+		if err != nil {
+			// Log but don't fail — one bad persona shouldn't break the whole review
+			continue
+		}
+
+		// Validate file size
+		if len(content) > MaxPersonaFileSize {
+			continue
+		}
+
+		persona, err := parsePersona([]byte(content), "repo:"+entry.Path)
+		if err != nil {
+			// Log but don't fail
+			continue
+		}
+
+		personas[persona.Name] = persona
+	}
+
+	return personas, nil
+}
+
+// LoadPersonaWithFallback loads a persona by name, checking the repo first, then built-ins.
+// This is the primary entry point for loading personas during a review.
+func LoadPersonaWithFallback(ctx context.Context, client RepoContentFetcher, owner, repo, name string) (*Persona, error) {
+	// Try repo personas first
+	repoPersonas, err := LoadRepoPersonas(ctx, client, owner, repo)
+	if err == nil {
+		if p, ok := repoPersonas[name]; ok {
+			return p, nil
+		}
+	}
+
+	// Fall back to built-in
+	return LoadBuiltinPersona(name)
+}
+
+// ListAllPersonas returns a merged list of available personas (repo + built-in).
+// Repo personas take precedence over built-ins with the same name.
+func ListAllPersonas(ctx context.Context, client RepoContentFetcher, owner, repo string) []string {
+	seen := make(map[string]bool)
+
+	// Built-ins first
+	for _, name := range ListBuiltinPersonas() {
+		seen[name] = true
+	}
+
+	// Repo personas override
+	repoPersonas, err := LoadRepoPersonas(ctx, client, owner, repo)
+	if err == nil {
+		for name := range repoPersonas {
+			seen[name] = true
+		}
+	}
+
+	names := make([]string, 0, len(seen))
+	for name := range seen {
+		names = append(names, name)
+	}
+	sort.Strings(names)
+	return names
+}
+
 // parsePersona parses persona data from JSON or YAML format.
 // Format is detected by the source file extension.
 func parsePersona(data []byte, source string) (*Persona, error) {