refactor: optimize file cleanups, propagate exit codes, and fix Makefile

- Unlink the temporary bootstrap launcher binary immediately after opening a read-only descriptor to it, then execute via `/proc/self/fd/<fd>` to ensure zero-disk footprint on execution.
- Unlink temporary `/tmp/resolvconf*` files immediately after successful bind-mounting over `/etc/resolv.conf`.
- Prune parent ephemeral profile directories when unpinning a namespace, leaving zero directories behind once empty.
- Propagate the exact exit status of the wrapped command to the host process using `errors.As` and `*exec.ExitError` instead of defaulting to exit code 1.
- Added E2E automated test `TestExitCodePropagation` to verify exit status delivery.
- Added the `$(BINARY)` target to `.PHONY` in the Makefile to delegate dependency tracking to Go's compiler cache, ensuring modified Go files are rebuilt during `make test`.

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