Progressive Tab Completion for File Paths

[mishudark]

1. Problem Statement

Current file path completion in Crush requires users to:

• Type the full path manually (error-prone, slow)

This creates a disjointed experience compared to modern shells like Fish, where Tab progressively completes into
directories without losing context.

Example:

1. Type @internal/u and highlight internal/ui/util using the ARROW UP key

2. Press TAB key

3. It is not possible to continue selecting internal files

2. User Story

As a developer using Crush to reference files in my project, I want to press Tab to accept a directory and continue
completing its contents, So that I can navigate deep paths efficiently without re-triggering completions.

3. Acceptance Criteria

AC1: Given I type @ and see completions, when I press Tab on a directory, then the path is inserted AND the menu stays
open showing that directory's contents.

AC2: Given I'm in continue mode after Tab, when I type additional characters, then the filter updates correctly treating
the full path (including slashes) as the query.

AC3: Given I see completions, when I press Enter, then the selection is accepted, the menu closes, and the file is
attached.

AC4: Given I Tab-complete a directory, when I continue typing, the completion menu remains open until I press Enter or
Escape.

[mishudark]

POC in #2167