Post-0.18 SDK polish and product decisions

[JoshuaPurtell]

Track the remaining small cleanup decisions after the 0.18 burn-down:

• resolve the research.Client transport name vs public.Client facade collision
• replace ProjectSelector's silent misc default with an explicit product decision
• remove dead testing/fake_backend.py fixtures
• settle .gitignore handling for AGENTS.md, CHANGELOG.md, and uv.lock
• consolidate the five duplicated CLI auth/URL helper blocks under cli/lib/
• decide whether customer CLI smoke commands stay public or move to a developer harness

Acceptance criteria:

• each item is either implemented or split into an owned issue with a concrete decision
• public naming/default changes include migration notes
• no exception-swallowing or hidden fallback is introduced