Settings -> Extensions offered an Install button even when extensions were
unconfigured, then dead-ended on 'Extensions not configured' because
install_extension required HERMES_WEBUI_EXTENSION_DIR to be set to an existing
directory. Default to a WebUI-managed root under STATE_DIR/extensions and
bootstrap it on first install so install just works with zero config; the env
var remains an optional override (never auto-created). Gallery-installed
extensions already auto-load via the runtime manifest, so the extension loads
on the next app-shell render with no further setup.
- api/extensions.py: _default_extension_root + _writable_extension_root;
_extension_root/_extension_root_status fall back to the managed default;
install_extension bootstraps via _writable_extension_root; dir-unavailable
warning only fires for an explicitly-set-but-broken env path.
- tests: update disabled-by-default expectations (configured=True, valid=False
pre-install) + new regression proving install bootstraps + auto-loads with
no env.
- docs/EXTENSIONS.md: document zero-config one-click install + env now optional.
- CHANGELOG [Unreleased].
