fix(setup): report account conflicts clearly and prepare 1.0.6

[escott-]

Summary

Signed-in setup commands printed “Signed in as” before verifying an existing machine account. A conflicting account or verification failure could then stop setup after that misleading success message.

Report sign-in success only after credential resolution, preserve existing credentials on non-interactive conflicts, and explain the fresh-command, interactive-confirmation and CI/environment recovery paths. This completes the installer portion of contextstream/contextstream#591; the matching documentation is already live. Prepare version 1.0.6 across Rust, npm and MCP Registry metadata.

Test plan

• Real CLI tests use an isolated temporary home and local mock API: conflicting accounts and API verification errors fail without sign-in success or credential changes.
• Both regression tests fail on unchanged main and pass with this fix.
• Release-contract tests: 12 passed.
• Formatting check passed; refreshed full CI must pass before merge.
• No real account credentials were used or changed. Publishing follows the protected release workflow.