Description

Fix local file tree blinking and reshuffling after connecting to an SSH session.

Root cause: When a remote SSH session is active, the remote server streams frequent
FileTreeEntryUpdated { Remote } events for every filesystem change on the remote host.
The previous code called rebuild_flattened_items() on each such event, which re-flattened
all roots (including local ones), causing the local file tree to re-render on every remote
filesystem change — visible as constant blinking/reshuffling.

Fix: Add an optional target_root parameter to the core rebuild_flatten_items_impl
method. When Some, only the specified root is re-flattened; all other roots keep their
existing items untouched. Three thin wrappers provide the public API:

• rebuild_flattened_items_for_root(path) — single-root rebuild
• rebuild_flattened_items() — full rebuild (all roots)
• rebuild_flattened_items_without(path) — full rebuild excluding a deleted path

Updated the following event handlers to use per-root rebuilds instead of full rebuilds:

• FileTreeEntryUpdated { Local } — rebuilds only the affected local root(s)
• FileTreeEntryUpdated { Remote } — rebuilds only the affected remote root
• RepositoryRemoved { Remote } — no-op rebuild (root already removed), avoids touching remaining roots

Skipping unchanged roots is safe because the rebuild is fully deterministic given the same
entry, expanded_folders, and item_states — re-flattening an unmodified root produces
an identical items list and selection index.

Testing

• cargo check -p warp --lib passes cleanly
• Verified the three updated event handlers and the rename flow (editing.rs) all route through rebuild_flatten_items_impl correctly

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode