chore: Bump version to 0.21.6

Updated all component versions:
- bd CLI: 0.21.5 → 0.21.6
- Plugin: 0.21.5 → 0.21.6
- MCP server: 0.21.5 → 0.21.6
- Documentation: 0.21.5 → 0.21.6

Generated by scripts/bump-version.sh

cmd/bd/version.go

@@ -11,7 +11,7 @@ import (
 
 var (
 	// Version is the current version of bd (overridden by ldflags at build time)
-	Version = "0.21.5"
+	Version = "0.21.6"
 	// Build can be set via ldflags at compile time
 	Build = "dev"
 )