Repository-focused file browser (floating or right pane)

[bastien-gallay]

What

A file browser for the focused repository, shown as a floating pane or a
right-side pane within the tab area.

Design-first, greenfield — no file-tree or explorer exists anywhere in the
app today (the closest thing, the Plans & memory list in crates/app/src/docs.rs,
is a flat fixed set of Markdown docs, not a tree).

To shape

• Tree vs list; lazy-loaded directories.
• Scope: repo root of the focused session's project.\n- Interactions: open in editor? reveal? drag into terminal? read-only?\n- Layout: floating overlay vs docked right pane, and how it coexists with the\n terminal/split panes.\n- Which crate owns the fs walk (a new adapter implementing a core port).\n\nRun /feature-torture before filing implementation issues.

[bastien-gallay]

Refactor prerequisite: #168 (C2 view/sidebar.rs) — the pane composition point will already be isolated.