Add skip_all option for checks and check collections

[rmm5t]

Summary

Adds a skip_all: true registration option for checks that should remain directly accessible without running as part of /okcomputer/all.

This also enables independently addressable groups of checks:

versions = OkComputer::CheckCollection.new("Versions")

OkComputer::Registry.register "versions", versions, skip_all: true
OkComputer::Registry.register "ruby_version", OkComputer::RubyVersionCheck.new, "versions"
OkComputer::Registry.register "app_version", OkComputer::AppVersionCheck.new, "versions"

The version checks are available through /okcomputer/versions and their individual endpoints, but are omitted from /okcomputer/all.

Implementation

• Adds a skip_all attribute to checks and check collections.
• Configures the default collection to omit entries marked skip_all.
• Keeps skipped entries available through registry lookup.
• Preserves normal behavior for named collections.
• Supports excluding individual checks as well as entire collections.
• Documents grouped checks and the new registration option.

Testing

• Added coverage for execution, lookup, text output, JSON output, and success status.
• Added coverage for skipped individual checks and collections.
• Full suite passes with 461 examples, 0 failures.

/cc @dgarwood

[emmahsax]

I can make a new release too after this is merged.

[rmm5t]

I can make a new release too after this is merged.

Thank you!