Wire up completion keyboard navigation and add AGENTS.md

- Tab/Enter confirms the selected completion item
- Up/Down arrows navigate the completion menu
- Escape dismisses the menu (via existing dismiss_lsp_overlays)
- Editor intercepts keys via completion_intercept_keys flag when menu is showing
- Add AGENTS.md with LLM-powered predictive autocomplete idea

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/src/code/editor/view/actions.rs

@@ -805,6 +805,28 @@ impl TypedActionView for CodeEditorView {
             return;
         }
 
+        // When completion menu is active, redirect navigation/confirm keys to the parent.
+        if self.completion_intercept_keys {
+            match action {
+                MoveUp => {
+                    ctx.emit(super::CodeEditorEvent::CompletionNavigateUp);
+                    return;
+                }
+                MoveDown => {
+                    ctx.emit(super::CodeEditorEvent::CompletionNavigateDown);
+                    return;
+                }
+                Tab | Enter => {
+                    ctx.emit(super::CodeEditorEvent::CompletionConfirm);
+                    return;
+                }
+                Escape => {
+                    // Let escape fall through to normal handling (which emits EscapePressed)
+                }
+                _ => {}
+            }
+        }
+
         match action {
             UserTyped(content) => self.user_insert(content, ctx),
             VimUserTyped(content) => {