Code browsing: project file tree and read-only file viewer #501

Idea

Add code browsing to the active project: a file tree for the project root and a read-only viewer that opens any file with syntax highlighting, line numbers, and a way to jump to a line or range. The viewer is a plain view of the working tree as it is on disk; it does not edit, and it does not interpret the agent's activity (that is a separate exploration, see the follow-up issue linked below).

Why

Today the only way to see project code inside Tcode is through what the agent shows: a command's output, an edit's diff hunk, or the finished-turn changed-files summary that opens a diff. There is no way to open an arbitrary file, look at surrounding context, or navigate the tree. Every client is a screen for the host (Principle 2), so browsing has to be served by the host and work the same on desktop, phone, tablet and browser; a phone reviewing a remote machine's code is the main case where an external editor is not available.

Scope

• A file tree for the project root, lazily listed by the host, respecting .gitignore for the default view with a toggle to show ignored files.
• A file viewer for text files: syntax highlighting through the existing syntect setup in crates/ui/src/highlight.rs, line numbers, wrapping toggle, and a "go to line / range" entry point other surfaces can call. Binary and very large files get a placeholder, not a render attempt.
• A protocol addition so remote clients read tree listings and file contents from the host, with size caps; nothing is read outside the project root.
• Entry points: a sidebar item per project, and the existing file links in the changed-files summary and diff panel open the file in the viewer at the relevant line.

Out of scope: editing, search across files, live following of the agent (follow-up issue), and anything that changes what the model sees or does (Principle 1).

Open questions

• Where the viewer lives in the shell: a tab next to the diff panel and preview panel, or a pane inside the thread view. Phone geometry decides this; check it with cargo run -p tcode-ui --example phone.
• Whether the tree stays cached on the host per project and invalidated by a watcher, or is listed on every expand.

Code context

Reviewed against main at 9a64fbae (2026-09-22).

• crates/ui/src/diff/view.rs: DiffPanel, the closest existing file-level render surface.
• crates/ui/src/chat/components/changed_files.rs: open_files handlers that would target the viewer.
• crates/ui/src/highlight.rs: existing highlighting.
• crates/ui/src/preview_panel.rs: a host-backed panel that already crosses the client/host boundary.