feat(persona): add role-based review personas (#51)

Implement role-based review personas that provide specialized review focus:
- Security: vulnerabilities, auth, secrets, injection attacks
- Architect: design patterns, code organization, API contracts
- Docs: documentation quality, API clarity, error messages

Changes:
- Add persona loading from JSON files and embedded built-ins
- Add --persona and --persona-file CLI flags (mutually exclusive)
- Add BuildPersonaSystemPrompt for persona-specific prompts
- Add FormatMarkdownWithDisplay for persona display names
- Update action.yml with persona and persona-file inputs
- Add comprehensive tests for all new functionality
- Document personas in README with examples

The persona system replaces the generic 'You are an expert code reviewer'
prompt with domain-specific identity, focus areas, ignore list, and
severity calibration. This reduces redundancy between multiple reviewers
and catches domain-specific issues that generic reviewers miss.

Closes #51

review/formatter.go

@@ -53,3 +53,48 @@ func GiteaEvent(verdict string) string {
 		return "COMMENT"
 	}
 }
+
+// FormatMarkdownWithDisplay formats a ReviewResult with separate display name and sentinel name.
+// displayName is used for the header title, sentinelName is used for the cleanup sentinel.
+// If displayName is empty, sentinelName is used for both.
+func FormatMarkdownWithDisplay(result *ReviewResult, displayName, sentinelName string) string {
+	var sb strings.Builder
+
+	// Use display name for header, or fall back to sentinel name
+	headerName := displayName
+	if headerName == "" {
+		headerName = sentinelName
+	}
+
+	if headerName != "" {
+		title := strings.ToUpper(headerName[:1]) + headerName[1:]
+		sb.WriteString(fmt.Sprintf("# %s Review\n\n", title))
+	}
+
+	sb.WriteString("## Summary\n\n")
+	sb.WriteString(result.Summary)
+	sb.WriteString("\n\n")
+
+	if len(result.Findings) > 0 {
+		sb.WriteString("## Findings\n\n")
+		sb.WriteString("| # | Severity | File | Line | Finding |\n")
+		sb.WriteString("|---|----------|------|------|--------|\n")
+
+		for i, f := range result.Findings {
+			sb.WriteString(fmt.Sprintf("| %d | [%s] | `%s` | %d | %s |\n",
+				i+1, f.Severity, f.File, f.Line, f.Finding))
+		}
+		sb.WriteString("\n")
+	}
+
+	sb.WriteString("## Recommendation\n\n")
+	sb.WriteString(fmt.Sprintf("**%s** — %s\n", result.Verdict, result.Recommendation))
+
+	if sentinelName != "" {
+		sb.WriteString(fmt.Sprintf("\n---\n*Review by %s*\n", headerName))
+		// Hidden sentinel for identifying this bot's reviews during cleanup
+		sb.WriteString(fmt.Sprintf("\n<!-- review-bot:%s -->\n", sentinelName))
+	}
+
+	return sb.String()
+}