Split packages/iterate/src/cli.ts (1,518 lines)

The package's remaining 1k-line-rule offender (flagged by #2063's thermo-nuclear review after the PR deleted the previous one). Natural seams visible today: OAuth/session management (~500 LOC — see tasks/iterate-auth-oauth-session-dedupe.md, which removes most of it), project resolution/setup, the chat launcher, and command registration. Do this after (or as part of) the auth dedupe so the split follows the real boundaries rather than inventing new ones.