fix(models): list models in pi's /model order, not by name (#783)

[agegr]

The route and ModelSelector both sorted models by display name, so a models.json provider lost the order its file gives (#783).

pi's own /model selector never sorts by name:

• with enabledModels set it lists the scope in the order the patterns are written (a glob expands in config order);
• the full list is sorted by provider id only, keeping registry (= file) order within each provider.

The name sort came in as a side bullet of #80 ("Sort model options alphabetically"); no issue asked for it.

• lib/model-scope.ts: new orderSelectorModels() gives pi's order; app/api/models/route.ts uses it.
• components/ModelSelector.tsx no longer re-sorts; it still groups by provider (first-appearance order).
• Differences from the TUI, kept on purpose: the current and default models are marked in place rather than lifted to the top, and a scoped list that alternates providers is grouped under provider headers.
• docs/agents/models.md notes the rule.

Tests: lib/model-scope.test.mjs, components/ModelSelector.test.mjs (+ a regex in components/AgentsConfig.test.mjs). Checked against the real SDK with a scratch models.json that file order is kept. tsc, eslint, npm test pass.

Closes #783

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