Adds support for repository-specific personas. When --persona is
specified, review-bot now:

1. Checks the target repo's .review-bot/personas/<name>.yaml directory
2. Falls back to built-in persona if not found in repo

This allows repos to define domain-specific personas (trading, regulatory,
etc.) or override built-in personas with project-specific rules, without
requiring changes to CI configuration.

Implementation:
- New review.PersonaFetcher interface for abstracting Gitea API access
- review.LoadRemotePersonas() with graceful fallback on 404
- review.MergePersonas() for combining remote and built-in personas
- giteaFetcher adapter in main.go to bridge gitea.Client

The feature follows a partial-success model: invalid YAML files or
network errors for individual persona files are logged and skipped,
allowing other valid personas to load.

Closes #60

@@ -199,0 +194,4 @@
+			var err error
+			persona, err = review.LoadBuiltinPersona(*personaName)
+			if err != nil {
+				slog.Error("failed to load persona", "persona", *personaName, "error", err)

@@ -0,0 +75,4 @@
+		}
+		if !isYAMLFile(entry.Name) {
+			continue
+		}

@@ -0,0 +47,4 @@
+	entries, err := fetcher.ListContents(ctx, owner, repo, path)
+	if err != nil {
+		// 404 is expected when repo doesn't have personas — return empty, not error
+		if isNotFoundError(err) {

@@ -199,0 +185,4 @@
+			// Assign empty map so the lookup below doesn't panic
+			remotePersonas = map[string]*review.Persona{}
+		}
+

@@ -0,0 +106,4 @@
+// MergePersonas combines remote and built-in personas.
+// Remote personas take precedence on name collision.
+// Returns the merged map and a list of persona names in sorted order.
+func MergePersonas(remote, builtin map[string]*Persona) (map[string]*Persona, []string) {

@@ -0,0 +155,4 @@
+// isNotFoundError checks if an error indicates a 404 response.
+// This is a simple string check to avoid importing the gitea package
+// (which would create a circular dependency).
+func isNotFoundError(err error) bool {