Keybase can't find the secret key, although it claims it's stored

```
% keybase status
...
Session: no session
is valid: yes
keys: unlocked

Key status:
stream: not cached
secret: stored
dev sig: cached
dev enc: cached
paper sig: not cached
paper enc: not cached
prompt: show
tsec: not cached
...
```

```
% keybase pgp sign -m test
▶ ERROR No secret key available
```

Can't export it either:

```
% keybase pgp export -s
▶ ERROR No matching keys found
```

Start by reproducing the discrepancy with `keybase status`, `keybase pgp sign -m test`, and `keybase pgp export -s`. Trace why status reports the secret key as stored while signing and export cannot find it. Done means the stored secret key is available to both PGP commands, with tests covering the reported state.

Written by the indexing model from the issue text.