toolchain remove: rejects the space-separated form that install and default both accept #284

mcpp toolchain remove only takes the @ spelling, while its two sibling subcommands take either. Nothing signals that, so the natural attempt fails with an error that reads like the family name itself is wrong:

$ mcpp toolchain remove gcc 15 --target x86_64-linux-musl
error: invalid spec 'gcc'

$ mcpp toolchain remove gcc@15 --target x86_64-linux-musl   # works (partial version resolves)
error: gcc@15 → x86_64-linux-musl is not installed

Reproduced on 0.0.107.

Cause

remove declares one positional; install and default each declare two (src/cli.cppm):

.subcommand(cl::App("install")            // 365
    .arg(cl::Arg("compiler")...)
    .arg(cl::Arg("version")...)           // ← second positional

.subcommand(cl::App("default")            // 377
    .arg(cl::Arg("spec").required())
    .arg(cl::Arg("version")...)           // ← second positional

.subcommand(cl::App("remove")             // 385
    .arg(cl::Arg("spec").help("<family>@<version>").required())
    // no second positional

So gcc lands in spec alone and fails to parse. Both install and default carry a comment explaining that the two spellings are equivalent — remove looks like it was simply missed.

Fix

Add the same second positional to remove and feed it through the same spec join. The help text (<family>@<version>) should then match defaults <family>[@<version>] (version may be partial), since partial versions already resolve here.

Worth a small e2e asserting all three subcommands accept both spellings, so they cannot drift apart again.