feat: add git user.name to actor fallback chain (#994)

- Insert git config user.name in fallback chain before final $USER default
- Consolidate duplicate actor resolution logic into single function
- Add BEADS_ACTOR as env var alias for MCP compatibility
- Add tests for actor resolution priority
- Update CONFIG.md with Actor Identity Resolution section

The new fallback order is:
  --actor flag > BD_ACTOR > BEADS_ACTOR > git user.name > $USER > "unknown"

Co-authored-by: Ohffs <ohffsnotnow@gmail.com>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/delete.go

@@ -6,7 +6,6 @@ import (
 	"encoding/json"
 	"fmt"
 	"os"
-	"os/exec"
 	"regexp"
 	"strings"
 
@@ -802,30 +801,6 @@ func uniqueStrings(slice []string) []string {
 	return result
 }
 
-// getActorWithGit returns the actor for audit trail with git config fallback.
-// Priority: global actor var (from --actor flag or BD_ACTOR env) > git config user.name > $USER > "unknown"
-func getActorWithGit() string {
-	// If actor is already set (from flag or env), use it
-	if actor != "" && actor != "unknown" {
-		return actor
-	}
-
-	// Try git config user.name
-	cmd := exec.Command("git", "config", "user.name")
-	if output, err := cmd.Output(); err == nil {
-		if gitUser := strings.TrimSpace(string(output)); gitUser != "" {
-			return gitUser
-		}
-	}
-
-	// Fall back to USER env
-	if user := os.Getenv("USER"); user != "" {
-		return user
-	}
-
-	return "unknown"
-}
-
 func init() {
 	deleteCmd.Flags().BoolP("force", "f", false, "Actually delete (without this flag, shows preview)")
 	deleteCmd.Flags().String("from-file", "", "Read issue IDs from file (one per line)")