Adds --dry-run to DiagnosticFlags, printing commands and file operations instead of executing them.

• DryRunRunner prints "Would run: {cmd}" instead of spawning processes
• Filesystem operations (create_dir_all, remove_dir_all) print "Would run: fs::{op}({path})"
• Build system detection and configure/build are skipped
• SetupPaths split from print_setup_complete to separate path resolution from display