fix(chat): support folder selection and visible workspace roots

[Is14w]

Linked issue

Closes #654

Summary

• Split the Composer attachment action into explicit Files and Folder choices. Desktop uses the native folder picker; WebUI accepts browser directory selection and imports the resulting directory tree.
• Mount selected or dropped external directories as workspace roots and show their aliases under the active project root in the right-side File Tree.
• Keep external roots read-only in the File Tree and avoid unsupported primary-workspace-only actions, while preserving their real paths for copy, reveal, and desktop open operations.
• Refresh File Tree roots after folder mount or manual refresh, and prevent delayed roots from a previous project appearing after a workspace switch.

Change scope

• Modules: agent-ui, agent-gui, agent-gateway/web, architecture docs
• Key paths:
  • crates/agent-ui/src/pages/chat/ChatComposerBar.tsx
  • crates/agent-ui/src/components/project-tools/file-tree/*
  • crates/agent-gui/src/pages/ChatPage.tsx
  • crates/agent-gui/src/pages/chat/hooks/useUploadZoneDrop.ts
  • crates/agent-gateway/web/src/app/{GatewayApp.tsx,GatewayAppView.tsx}
  • crates/agent-gateway/web/src/app/hooks/{useDirectoryDropActions.ts,usePendingUploads.ts}
  • crates/agent-gateway/web/src/lib/directoryDrop.ts
  • docs/architecture/{gui.md,webui.md}

Screenshots / preview

Draft pending a manual desktop capture before review:

1. Open a conversation with a workspace, choose + -> Folder, and select a directory outside the workspace.
2. Open the right-side File Tree and expand the selected folder's alias beneath the project root.
3. Show that the external root is browseable but has no write or mention actions.

Verification

• pnpm --dir crates/agent-ui typecheck
• node --test crates/agent-gateway/test/webui/directory-import.test.mjs (5 passed)
• node --test crates/agent-gui/test/chat/execution-mode-model-picker.test.mjs crates/agent-gui/test/settings/right-dock-model.test.mjs (39 passed)
• node --test crates/agent-gui/test/workspace-file-tree-row.test.mjs (2 passed)
• pnpm --dir crates/agent-gui build
• pnpm --dir crates/agent-gateway/web build
• git diff --check upstream/main...HEAD

Earlier full-suite verification before this documentation-only follow-up:

• pnpm --dir crates/agent-gateway/web test (633 passed)
• pnpm --dir crates/agent-gui test:frontend (2,613 passed; 5 pre-existing failures: four Windows CRLF-sensitive source-text assertions and one Rust/TypeScript builtin-provider preset synchronization assertion, none in paths modified by this PR.)

Pre-submit checklist

• A requirement issue is linked (or this is a trivial fix that needs no issue, as explained in the summary).
• Synced with the target branch; no merge conflicts.
• The change is focused, with no unrelated modifications.
• No secrets, tokens, or personal data included.
• Docs are updated for changes affecting user behavior, deployment, or configuration.