Reduce dead exports in mode resolver API surface

[clopca]

Context

getDefaultModeConfig in src/modes/resolver.ts is currently exported but unused.

Problem

Unused exports increase public surface area and maintenance burden.

Proposed direction

• Remove the export if no external caller needs it
• If it is intended as public API, add at least one production usage and test coverage
• Keep resolver API minimal and intentional

Acceptance criteria

• No unused exported resolver helpers
• Lint/typecheck pass
• Resolver public API is documented and intentional