fix: treat lone dash as positional during command discovery

[ibrahimypr]

When a runnable root command accepts positional arguments and has a child subcommand, root - child currently executes child with - as its argument. Replacing - with an ordinary filename keeps execution on the root. Enabling TraverseChildren also keeps the lone dash on the root, consistent with pflag treating - as a positional argument.

Use the existing isFlagArg predicate in command discovery and matched-command removal. A lone dash can then stop subcommand discovery like other positional arguments. If an application explicitly defines - as a command name or alias, its command token is removed normally. Flag values and the -- terminator retain their existing handling.

Validation on macOS arm64 with Go 1.25.3:

• The strip-flags case and default-dispatch regressions failed before the fix.
• Added coverage for lone dashes before and after a subcommand, after a boolean flag, as a flag value, around --, and as a command name or alias.
• go test ./... passes.
• make all (formatting and full tests) and git diff --check pass. The optional golangci-lint binary is not installed locally.

AI assistance: OpenAI Codex helped implement and test this change.