Fix: add missing yellow color function in init.go

The yellow color function was referenced but not defined, causing
CI build failures.

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Co-Authored-By: Claude <noreply@anthropic.com>

cmd/bd/init.go

@@ -353,6 +353,7 @@ With --no-db: creates .beads/ directory and issues.jsonl file instead of SQLite
 
 		green := color.New(color.FgGreen).SprintFunc()
 		cyan := color.New(color.FgCyan).SprintFunc()
+		yellow := color.New(color.FgYellow).SprintFunc()
 
 		fmt.Printf("\n%s bd initialized successfully!\n\n", green("✓"))
 		fmt.Printf("  Database: %s\n", cyan(initDBPath))