## Why
Skill mentions and shell parameters both use a `$` prefix. Numeric and special
shell parameters should not open or displace completion popups, while loaded
skills can legitimately have digit- or hyphen-leading names.
## What changed
- Classify `$` queries as skill candidates, shell variables, definite shell
parameters, or ambiguous shell-like names.
- Offer completion for an ambiguous name only when it fuzzy-matches a bindable
loaded mention, reusing the filtered catalog when the popup opens.
- Keep definite shell forms such as `$1`, `$12`, `$-`, and `$_` out of mention
completion and preserve completion targets elsewhere in the composer.
## Testing
Added unit and snapshot coverage for positional and special parameters,
digit- and hyphen-leading skills, uppercase shell variables, unbindable
qualified skills, and competing completion targets.
GitOrigin-RevId: 8bb64cb5ca0fa8233c7fadb11a53678d57eb814c
Charlie Marsh
b6f1cfa3f3