`keybase login` incorrectly implies that one needs to upload one's private key to keybase.io

(Old title: "`keybase login` requires upload of private key to keybase?" -- see comments.)

Am I reading this correctly that I am required to upload my _private key_ to keybase in order to log in locally? This is pretty weird. GPG is installed and working, and the gpg agent was even unlocked for one attempt at this (no effect).

```
$ keybase login
How would you like to sign this install of Keybase?

(1) Use an existing device
(2) Use a paper key
(3) Use my Keybase passphrase
(4) Use GPG
Choose a signing option: 3
Your keybase username or email address: timmc
▶ ERROR in Login

Sorry, you have a PGP private key that Keybase doesn't have a copy of.
You need to prove you're you. Any of these will unlock this computer:

- reset your account and start fresh: https://keybase.io/#account-reset
- install GPG and put your PGP private key on this machine and try again
- go back and provision with another device or paper key
```

Start by reproducing `keybase login` with the signing choices shown in the issue, especially the GPG path and the resulting error. Trace where these prompts and errors are assembled; done means the login flow clearly explains the local-key requirement without implying that a private key must be uploaded to Keybase.

Written by the indexing model from the issue text.