Add TypeToolkit/PreferNotNil cop

[Morriar]

What

Adds an autocorrecting TypeToolkit/PreferNotNil RuboCop cop that replaces Sorbet's T.must with Type Toolkit's .not_nil!.

# before
value = T.must(fetch_value)

# after
value = fetch_value.not_nil!

The cop is enabled by default.

Why

Type Toolkit's not_nil! provides the same non-nil assertion while producing a more useful UnexpectedNilError when the assertion fails. It also supports Core's migration away from runtime Sorbet constructs.

Autocorrection details

The autocorrection preserves Ruby semantics and syntax for:

• binary and logical operators
• conditionals, ranges, and assignments
• command-style calls and calls with blocks
• defined?, yield, and super
• string interpolation
• multiline calls, including comments and trailing commas
• nested T.must calls across RuboCop correction passes

Splat arguments are not corrected because they do not have a safe receiver-style equivalent.