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/tree-picker-state.ts

@@ -44,6 +44,7 @@ export type PickerCommand =
   | 'collapse-all'
   | 'toggle-check'
   | 'select-all-visible'
+  | 'toggle-select-all-visible'
   | 'select-none'
   | 'clear-transient-hint'
   | 'search-start'
@@ -228,6 +229,17 @@ export function isAncestorChecked(nodeId: string, checked: Set<string>, byId: Ma
   return ancestorsOf(nodeId, byId).some((ancestorId) => checked.has(ancestorId));
 }
 
+export function hasPartialChildren(
+  nodeId: string,
+  checked: Set<string>,
+  byId: Map<string, TreePickerNode>,
+): boolean {
+  if (checked.has(nodeId) || isAncestorChecked(nodeId, checked, byId)) {
+    return false;
+  }
+  return descendantsOf(nodeId, byId).some((descendantId) => checked.has(descendantId));
+}
+
 function checkedAncestor(nodeId: string, state: PickerState): TreePickerNode | null {
   for (const ancestorId of ancestorsOf(nodeId, state.byId)) {
     if (state.checked.has(ancestorId)) {
@@ -350,6 +362,16 @@ export function selectNone(state: PickerState): PickerState {
   return cloneState(state, { checked: new Set(), transientHint: null });
 }
 
+function toggleSelectAllVisible(state: PickerState): PickerState {
+  const next = selectAllVisible(state);
+  const unchanged =
+    next.checked.size === state.checked.size && [...next.checked].every((id) => state.checked.has(id));
+  if (unchanged && state.checked.size > 0) {
+    return selectNone(state);
+  }
+  return next;
+}
+
 function setExpanded(state: PickerState, nodeId: string, value: boolean | 'toggle'): PickerState {
   const expanded = new Set(state.expanded);
   const nextValue = value === 'toggle' ? !expanded.has(nodeId) : value;
@@ -487,6 +509,8 @@ export function reducer(state: PickerState, cmd: PickerCommand, now = Date.now()
       return { next: toggleChecked(state, state.cursorId, now), effect: null };
     case 'select-all-visible':
       return { next: selectAllVisible(state), effect: null };
+    case 'toggle-select-all-visible':
+      return { next: toggleSelectAllVisible(state), effect: null };
     case 'select-none':
       return { next: selectNone(state), effect: null };
     case 'clear-transient-hint':