docs(cli): add help text for positional platform arguments

[glenlouis8]

What

• Add help="Target platform" to the required positional platform argument on pn run, pn logs, pn build, and pn app-id.

Why

• pn <cmd> --help listed {android,ios} under "positional arguments" with no description, while every flag on the same subcommand is described. doctor and devices already describe their platform argument; these four did not.

How (brief)

• One kwarg per add_argument("platform", ...) call in _build_parser() in src/pythonnative/cli/pn.py. Wording kept identical across the four and consistent with the existing doctor / devices strings.
• Extended the existing --help output tests (test_cli_run_help_lists_flags, test_cli_build_help_lists_debug) in tests/test_cli.py to assert the new text appears.

Testing

• ./scripts/check.sh passes.

Risks/Impact

• Help text only. No behavior change.

Docs/Follow-ups

• None.

Closes #55