feat: add testtrack show to read a split's remote weights

[bestdan]

What

Adds a read-only testtrack show <split> command that prints the current variant weights of a split / feature gate from the remote TestTrack server, without modifying the local schema.

Why

Reading live weights previously required testtrack sync (which mutates the local schema), the admin UI, or a raw curl. show is a side-effect-free lookup so devs can check a feature gate's current weights directly.

Usage

TESTTRACK_CLI_URL=https://tt.example.com testtrack show my_app.my_feature_enabled
# my_app.my_feature_enabled
#   false  100%
#   true     0%

--json emits the raw weights map for scripting. Works against both a real server and a local testtrack server.

Notes

• GETs api/v2/split_registry (no .json suffix) so it resolves against both the real Rails server and the bundled fakeserver (which registers the suffix-less route). Verified live against a demo environment.
• Reuses the existing serializers.RemoteRegistry; no new dependencies.
• Tests cover human/JSON formatting, sorted output, the not-found error, and the missing-TESTTRACK_CLI_URL path.