[FEATURE] ! shell mode: filesystem tab completion

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

When typing ! in an interactive Claude Code session, Tab only completes from command history. Filesystem completion (paths, directories, filenames) does not work, unlike a regular terminal.

Proposed Solution

Tab after ! should behave like a normal shell prompt. The input handler for ! mode could delegate Tab keypresses to the same completion engine that already powers @ file path completion.

Alternative Solutions

N/A - the @ completion infrastructure already exists in Claude Code, so this is an extension of existing behavior rather than a new system.

Priority

Low - Nice to have

Feature Category

CLI commands and flags

Use Case Example

! cat src/com<Tab> → src/components/
! ls ~/Do<Tab> → ~/Documents/ ~/Downloads/
! git diff src/<Tab> → src/index.ts src/utils.ts
! cd ../pro<Tab> → ../project-name/

Additional Context

Related: #40503 (Tab completion for CLI flags - different scope, outside the session).
The @ prefix already supports Tab path completion inside sessions, so the mechanism is in place.