Provide long help via rv help, remove per-command help subcommands

[carl]

Fixes #431.

What this does

Help is now requested in two consistent ways:

• rv … -h / rv … --help show short help (unchanged)
• rv help [NAME] shows long help for the targeted command

The per-command rv NAME help forms (e.g. rv ruby help pin) are removed by disabling the auto-generated help subcommand on each command group (ruby, tool, cache, self, shell), so there is a single way to ask for full help. The top-level help subcommand is preserved, so rv help ruby pin still works.

This addresses the two open checkboxes in #431:

• rv help [NAME] shows long help
• rv NAME help removed

Before / after

Invocation	Before	After
rv … -h / --help	short	short (unchanged)
rv help [NAME]	long	long (now covered by a regression test)
rv ruby help pin	printed help	removed → errors
rv ruby -h command list	listed help	no longer lists help
rv help ruby pin (top level)	worked	still works

Implementation

Five one-line additions of #[command(disable_help_subcommand = true)] to the command-group arg structs. clap's help subcommand renders long help (write_long_help) while -h/--help are mapped to HelpShort, so the long-vs-short routing already follows from keeping the top-level help subcommand and removing the nested ones.

Tests

crates/rv/tests/integration_tests/main.rs:

• test_help_subcommand_shows_long_help — rv help expands enum descriptions; rv -h does not
• test_help_subcommand_shows_nested_command_help — rv help ruby pin still prints the command's help
• test_help_subcommand_removed_from_command_groups — rv {ruby,tool,cache,self} help → unrecognized subcommand 'help'
• test_help_subcommand_removed_from_shell — rv shell help fails
• test_help_not_advertised_as_command_group_subcommand — help no longer appears in the -h command listing for ruby/tool/cache/self

Each behavior-changing test was confirmed to fail against the pre-change source. Full suite (cargo nextest run -p rv), cargo fmt --all -- --check, and cargo clippy -p rv --all-targets all pass.

Notes for reviewers (out of scope / open question)

1. Leaf positional swallows help. On commands whose leaf takes a positional, help is consumed as a value rather than rejected: rv ruby pin help pins the version to the literal "help", and rv shell help errors as invalid value 'help' for '[SHELL]' rather than unrecognized subcommand 'help'. These aren't help subcommands, so they're untouched by this change. Happy to address separately if you'd like them to error explicitly.
2. No long_about prose authored. This PR makes rv help [NAME] route to long help, but long help only differs visibly where an arg carries rich metadata (e.g. --color's enum descriptions). If you want genuinely richer per-command long help, that's a larger, separate effort I left out here. Let me know if it should be part of this issue.