Pitch

opencode parity. Some users want to scan the workspace without leaving the TUI. `Ctrl+E` toggles a left-side file tree (similar to opencode's "explorer" pane) showing:

• Workspace tree, gitignore-aware (uses our existing `ignore::WalkBuilder` walker).
• Cursor + Up/Down nav, Enter opens the file in the transcript as a read-only preview.
• `r` re-runs `grep_files` against the cursor's path.

Layout: when toggled, sidebar layout becomes `[file tree | chat | sidebar]` with sidebar collapsing if the terminal is < 160 cols.

Implementation sketch

• New `tui/file_tree.rs` widget.
• Lazy-load directory contents on first expand.
• File preview goes through the existing `read_file` tool, rendered as a Syntax-highlighted code cell.

Acceptance

• Ctrl+E toggles the pane; second press hides it.
• Tree respects .gitignore / .ignore / .git/info/exclude.
• Enter opens the file as a read-only transcript cell.
• No regression in narrow terminals (< 160 cols → sidebar collapses, file tree stays).