Support help subcommand in addition to --help flag

[schickling-assistant]

Problem

Most CLI frameworks (Cobra, commander.js, git) automatically provide a help subcommand (e.g. mycli help subcmd) in addition to the --help flag. Currently @effect/cli only supports --help/-h flags.

The help subcommand pattern is valuable because:

• Unambiguous — tool help X is always a read-only help request. With flags, --help can be consumed as a value by a preceding option (e.g. tool commit -m --help might create a commit with message "--help" depending on the parser).
• POSIX/GNU convention — users expect tool help to work.
• Tooling-friendly — command authorization systems (e.g. proxies that allow/deny CLI operations) can trivially distinguish help ... as read-only without parsing flags, which is harder with --help embedded anywhere in the argument list.

Requested behavior

• mycli help — shows top-level help (equivalent to mycli --help)
• mycli help subcmd — shows subcommand help (equivalent to mycli subcmd --help)
• mycli help nonexistent — shows an error listing available commands

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Created on behalf of @schickling