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/main_daemon.go

@@ -64,17 +64,8 @@ func signalOrchestratorActivity() {
 	// Build command line from os.Args
 	cmdLine := strings.Join(os.Args, " ")
 
-	// Determine actor (use package-level var if set, else fall back to env)
-	actorName := actor
-	if actorName == "" {
-		if bdActor := os.Getenv("BD_ACTOR"); bdActor != "" {
-			actorName = bdActor
-		} else if user := os.Getenv("USER"); user != "" {
-			actorName = user
-		} else {
-			actorName = "unknown"
-		}
-	}
+	// Determine actor (uses git config user.name as default)
+	actorName := getActorWithGit()
 
 	// Build activity signal
 	activity := struct {