feat(ui): /model with no arg opens an interactive picker

[esengine]

Closes #371.

Why

/model <id> required typing the model id by hand. New users in the issue thread report nearly closing the app on first use because there was no obvious way to switch — /models only listed available ids read-only, with no built-in way to act on what you saw.

What changes

Bare /model (no arg) now opens an interactive modal picker. /model <id> typed entry continues to work for users who know what they want.

• src/cli/ui/ModelPicker.tsx — new component, mirrors SessionPicker shape:

  • ↑↓ navigate, ⏎ confirm, [r] refresh catalog, esc cancel
  • Current model marked with a · current tag
  • Each row shows the existing modelBadgeFor() pill (flash / pro / r1) so the kind is at-a-glance
  • Seeds from the live DeepSeek catalog (useSessionInfo already fetches on mount); falls back to the four known ids when the catalog hasn't loaded yet so the picker isn't empty on first open
  • Current model is always included in the list, even if the API didn't return it (handles stale catalog / custom ids)

• src/cli/ui/slash/types.ts — adds openModelPicker?: boolean to SlashResult, parallel to existing openSessionsPicker / openMcpHub flags.

• src/cli/ui/slash/handlers/model.ts — bare /model returns { openModelPicker: true } instead of the prior info-only hint.

• src/cli/ui/App.tsx — pendingModelPicker state, slotted into the modalOpen guard and the modal-stack JSX. On select: loop.configure({ model: id }) + dispatches session.model.change (the event added in Model badge on assistant card still shows flash after switching to pro #372) so the next card pill reflects the new selection.

Tests

• tests/ui-model-picker.test.tsx — 7 new render tests: catalog list, current marker, loading hint, fallback when null, empty-catalog hint, current id always included, keybind footer.
• tests/slash.test.ts — updated the bare-/model case from "shows hint" to "returns openModelPicker: true" (existing behavior changed).

Test plan

• npm run verify — 134 files / 2152 passed / 1 skipped
• Manual: /model opens the picker, ↑↓ navigates, ⏎ selects, the next assistant card pill reflects the new model
• Manual: /model deepseek-v4-pro typed entry still works (no picker shown)
• Manual: open picker before catalog loads → fallback list visible; press [r] to refresh