[tylinux]

Summary

• Adds a Files entry to the side pane ("Open tab" launcher and the "+" menu) on desktop and web. It opens a singleton files tab that renders the existing WorkspaceFileTree for the current workspace (lazy expansion, search, changed-only filter, git status, context menu).
• Clicking a file opens it in a code-viewer tab next to the Files tab, so preview supports whatever the code viewer already does: text/code, markdown, images, audio/video, PDF, PPTX. No new preview implementation, service, protocol or persistence.
• WorkspaceFileTree.onClose is now optional; when absent the "Back to tasks" button is not rendered (it only makes sense in the sidebar overlay).
• files is a workspace-global tab type (like git / developer-tools): one tab per workspace, visible from every conversation in it; reopening activates the existing tab.

Spec: docs/specs/side-pane-file-browser.md

Test plan

• pnpm typecheck passes
• pnpm lint: 0 errors (80 pre-existing warnings)
• pnpm architecture:check --changed: 0 violations
• New unit tests (packages/ui/test/sidePaneFilesTab.test.ts, 4 pass): launcher order incl. web without embedded browser, single-tab activation, visibility across conversations. Run with pnpm exec tsx --tsconfig packages/ui/tsconfig.json --test packages/ui/test/sidePaneFilesTab.test.ts
• Manual web E2E (headless Chrome) against an isolated local server and against https://nex.tylinux.cn: launcher shows Files, tree lists the workspace, no "Back to tasks", clicking a .md opens a rendered code-viewer tab, Files tab stays
• Desktop (Electron) not exercised
• No automated E2E scenario added yet (the above was a one-off script)

Note: pnpm fmt:check already fails on 139 files on main; none of the lines changed here are newly flagged.