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/comments.go

@@ -4,7 +4,6 @@ import (
 	"encoding/json"
 	"fmt"
 	"os"
-	"os/user"
 	"strings"
 
 	"github.com/spf13/cobra"
@@ -132,20 +131,10 @@ Examples:
 			commentText = args[1]
 		}
 
-		// Get author from author flag, BD_ACTOR var, or system USER var
+		// Get author from author flag, or use git-aware default
 		author, _ := cmd.Flags().GetString("author")
 		if author == "" {
-			author = os.Getenv("BD_ACTOR")
-			if author == "" {
-				author = os.Getenv("USER")
-			}
-			if author == "" {
-				if u, err := user.Current(); err == nil {
-					author = u.Username
-				} else {
-					author = "unknown"
-				}
-			}
+			author = getActorWithGit()
 		}
 
 		var comment *types.Comment