[meta] DX and AI UX before 1.0

Help, errors, completion, and agent discoverability that should land before the first major.

Sub-issues are the work. Do not expand this into OAuth, MCP, XDG paths, or --debug; those are already tracked.

Related, not children: #625 quickstart, #316 --debug, #422 MCP, #574 XDG, #623 CAT output.