Web Shell Composer Add Menu Implementation Plan

Design: docs/design/web-shell/web-shell-composer-add-menu.md

Scope

Add an opt-in + toolbar menu to the Web Shell composer without changing daemon APIs or message contracts.

Decisions

• Reuse the toolbar action lists; addMenu stays absent from embedder defaults.
• Append addMenu to standalone defaults and opt split-view panes in explicitly without removing existing controls.
• Keep five one-line top-level submenu entries: Add file, Reference file, Extensions, MCP, and Skills.
• Add file chooses attachment or workspace upload before opening the native picker.
• Reference file opens on the workspace root, supports folder browsing, and uses workspace-wide glob search for name queries.
• Share the existing built-in @ providers instead of duplicating their availability, escaping, caching, or insertion rules.
• Keep MCP references at server scope because resource references are not consumed on the Web Shell backend path.
• Append reference chips without replacing selected draft text; prepend skills without replacing the draft or existing chips.
• Support both CodeMirror and the touch textarea backend.
• Keep menu navigation pointer-only; add no feature-specific keyboard handlers.

Implementation

• Extract built-in reference providers and helpers to useAtMentionSources.ts while preserving useAtMentionMenu.ts behavior.
• Add the opt-in toolbar action and standalone-host enablement.
• Add the + trigger and Add file destination submenu.
• Add root folder browsing and literal-safe workspace file search.
• Add extension and MCP server submenus.
• Add bounded, scrollable Skills submenu and prepend behavior.
• Add English and Chinese copy.
• Keep the search input mounted when focused and let an unfocused hover submenu close normally.
• Preserve @ search routing while enabling directory drill-in only in the Add menu.
• Distinguish loading, empty, and failed submenu states without custom keyboard navigation.
• Add focused unit and integration coverage.

Verification

• Final regression files: 7 files, 385 tests passed.
• Full Web Shell suite: 204 files, 4374 tests passed.
• ESLint passed for all touched TypeScript files.
• Prettier and git diff --check passed.
• Real-device touch flyout check remains manual, as called out in the design.

Repository-wide typecheck is currently blocked by unrelated pre-existing CLI session-PR errors, a duplicate property in a core test, and the approval-mode type error in packages/web-shell/client/App.tsx.