fix(registry): stop dropping the first native arg on NoId commands, and disambiguate optional-id vs. passthrough args after "--" (#159)

[BlueLabelscotch]

Description:

buildCtx's HasArgs/BuiltinFlags.Set logic decided whether to strip args[0] (assuming it was a leftover ) by re-checking vspec.RequiresId || vspec.AllowsId, instead of tracking whether an id was actually assigned. This caused two bugs:

1. NoId commands lost their first arg. For RequiresId verbs with no_id: true and no allows_id (e.g. execute artifact:pip_install), the id-assignment branch correctly skips consuming args[0] — but the stripping logic still assumed it had, silently dropping the first native arg. harness execute artifact:pip_install -- -r requirements.txt invoked pip with just requirements.txt, not -r requirements.txt.
2. Optional-id + native-args ambiguity. For a verb where the id is optional (AllowsId) alongside has_args: true, args[0] could be a passthrough token from after -- (e.g. -r) that only looks like a flag, indistinguishable by position alone from a real id.

Fix: introduce consumedIdArg, set to true only at the exact point ctx.Id is actually assigned from args[0], and use it (instead of re-deriving the condition) for the downstream stripping logic. For the optional-id branch, resolve the ambiguity with cmd.Flags().ArgsLenAtDash() — the id is only real if it precedes a literal -- (or there's no -- at all); if -- is the very first token, nothing was given as an id, regardless of how many flags preceded it.

Fixes#159