Replace the overlapping usage and cost model with canonical billing
primitives centered on ModelRef, ModelHandle, TokenCounts, and
BilledModelUsage. This also renames the public API and web surface from
usage to billing, removes compatibility aliases, and normalizes provider
usage adapters onto the shared billing vocabulary.
Aligns naming with the convention that "Config" is for file-level configuration
while "Options" and "Settings" describe runtime parameters. Also applies
rustfmt formatting fixes in web_auth.rs.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Add shared twin scenario helpers and use them to cover OpenAI-backed
CLI, agent parity, workflow, and exec integration paths. This brings the
worktree implementation back into the main checkout as a single commit.
