feat(cli): enforce required database selection and improve tree-picker UX (#86)

* feat(cli): enforce required database selection and improve tree-picker UX

- Require at least one database driver via prompt `required: true` instead of
  looping on empty selection; remove the now-dead retry/back-on-empty branch.
- Surface the recommended option with a "(recommended)" hint in the depth and
  query-history prompts.
- Tree picker: add `◧` partial glyph for parents whose descendants are checked,
  and make `a` toggle select-all-visible / select-none.

* fix(cli): drop unused export from tree-picker toggleSelectAllVisible

Knip flagged the export as unused; the function is only consumed by the
internal reducer via the 'toggle-select-all-visible' command, so demote
it to a module-local helper to keep CI's dead-code check green.

* test(cli): drop empty-selection warning assertion from setup test

The empty-selection retry/warning loop in `chooseDrivers` was removed in
favor of `multiselect`'s `required: true`, so the legacy warning string
is unreachable. Update the test to assert the simpler back-from-selection
return-to-embeddings flow.

packages/cli/src/setup-database-context-depth.ts

@@ -46,12 +46,16 @@ async function chooseSetupDatabaseContextDepth(input: {
   const options =
     recommended === 'deep'
       ? [
-          { value: 'deep', label: 'Deep: AI descriptions, embeddings, relationships, slower' },
+          {
+            value: 'deep',
+            label: 'Deep: AI descriptions, embeddings, relationships, slower',
+            hint: 'recommended',
+          },
           { value: 'fast', label: 'Fast: schema only, no AI, quickest' },
           { value: 'back', label: 'Back' },
         ]
       : [
-          { value: 'fast', label: 'Fast: schema only, no AI, quickest' },
+          { value: 'fast', label: 'Fast: schema only, no AI, quickest', hint: 'recommended' },
           { value: 'deep', label: 'Deep: AI descriptions, embeddings, relationships, slower' },
           { value: 'back', label: 'Back' },
         ];