v0.3.1

Patch release. Fixes a critical bug in mainwp init (and
mainwp config set ...) where the active profile never received
its url, api_key, and api_path.

Root cause was two compounding bugs in lib/config.sh:

1. mainwp_config_set_field built a jq expression that read
   .url from the document root instead of writing to
   .profiles[$p].url. The new value was passed via --arg v
   but never referenced in the expression, so every set call
   wrote the literal string null to the config file.
2. mainwp_config_set_active declared local cfg="$1" and
   required a positional argument, but the init command called
   it with no arguments. Combined with the broken set_field,
   the visible result was set -u: $1: unbound variable from
   inside the function followed by a config file containing
   only {"active": "default"}.

After the fix, mainwp init stores the profile correctly and
mainwp config get round-trips the values.

Also: tests/smoke.sh now sandboxes $HOME to a temp dir for
the duration of the run. A previous run of the test suite had
polluted the real ~/.config/mainwp/config.json with the
broken null payload (same root cause); that file is wiped.
The sandbox makes the suite safe to run on a developer machine
that has a real MainWP Dashboard.

45 smoke tests, all passing. Upgrade with
`brew update && brew upgrade mainwp`.