Weave Router model selection

When the user invokes $router-models, show which models this
installation lets the router pick from, and change that selection when asked.
This is the same list — and the same stored setting — as the checkboxes on the
router dashboard's settings page.

Start by running:

npx --package @weave-os/router -y -- weave-router models --codex{{SCOPE}}

That prints every deployed model grouped by provider, with [x] for models the
router may pick and [ ] for models it may not. Present it back as a compact
checklist in that same [x] / [ ] form, keeping the provider grouping and the
exact model ids — the user selects models by id.

Then:

• If the user named models or providers, work out whether they want them on or
  off from how they phrased it, and apply it with
  weave-router models enable <id>... --codex{{SCOPE}} or
  weave-router models disable <id>... --codex{{SCOPE}} (add providers before
  enable/disable to switch a whole provider). Several ids can go in one
  call. Then re-run the list and show the result.
• If they named nothing, stop after the list and ask which ones to change.
  Never change anything they did not ask for.

Other things they might ask for:

• Rank models by preference:
  weave-router models prefer <id> <id>... --codex{{SCOPE}} (order matters), or
  weave-router models prefer clear --codex{{SCOPE}} to drop the ranking.
• Providers only: weave-router models providers --codex{{SCOPE}}.

If the command reports that this router doesn't expose model selection, that's a
Weave-hosted router: model selection belongs to the whole organization there, so
tell the user to change it at https://router.workweave.ai/dashboard/settings —
don't try to work around it. Its listing carries no on/off state, so present it
as a plain list; don't infer which models are enabled.

Disabling a model takes effect on the router's next routing decision; no restart
is needed.