docs: clarify headless OAuth process lifetime

[asim48-ctrl]

Summary

• Document that mcporter auth --no-browser still needs the CLI process to stay alive until the loopback callback completes
• Call out process managers that capture stdout and then clean up the process group, leaving no callback listener
• Add persistent-terminal/tmux/nohup guidance in the README, ad-hoc auth docs, and known issues

Refs #177.

Validation

• git diff --check
• Not run: full docs/test gate (docs-only change; local Bun/pnpm validation avoided to keep this lightweight)