cli: add long help text and command registry

[abraithwaite]

Summary

• Adds WithLong(text) CmdOption for multi-line help text shown in --help
• Adds cli.Register() / cli.Registered() for self-registering commands from outside main (supports build tags)

Long Help Text

cli.Command("up", "Launch a forge pod", &UpCmd{},
    cli.WithLong(`Launch a new forge pod for the given branch.
The pod runs Claude Code in headless mode with the given prompt.`),
)

Short description still used in parent command listings. Long text shown in --help output.

Command Registry

// In a separate package, conditionally included via build tags:
//go:build debug
func init() {
    cli.Register(cli.Command("debug", "Debug tools", &DebugCmd{}))
}

// In main:
app.AddCommand(cli.Registered()...)

Test plan

• Long help text appears in --help output
• Short desc still used in parent listings
• Register/Registered accumulates commands
• Registered returns a copy