container exec accepts --ulimit but ignores it

`container exec --help` lists `--ulimit Set resource limits (format: =[:])`, and the flag parses without error, because `ContainerExec` embeds `Flags.Process`. Nothing on the exec path ever reads `processFlags.ulimits` though: `grep -rn '\.ulimits' Sources` hits only `Parser.swift`, which is the run/create path, so exec just inherits the init process rlimits and the value you passed is dropped silently. The guest already honors per-process rlimits on exec (containerization's `vmexec/ExecCommand.swift` calls `App.setRLimits(rlimits: process.rlimits)`), so the plumbing is there and only the CLI side is missing. `container machine run` has the same gap. A PR follows.

Start with ContainerExec and its embedded Flags.Process, then compare the existing ulimits handling in Parser.swift with containerization's vmexec/ExecCommand.swift. Run container exec --help and exercise the relevant commands; done means the supplied limits are applied for both container exec and container machine run instead of being silently dropped.

Written by the indexing model from the issue text.