feat(shell): multi-line auto-growing workspace composer

[bengidev]

Summary

• Configure the workspace composer for auto-growing multi-line input with submit_on_enter, so Enter submits and Shift+Enter inserts a newline.
• Remove the fixed composer input height and bottom-align the send button so the field can expand with content (up to 6 lines).
• Update the composer helper text to document the Shift+Enter newline shortcut.

Test plan

• Open the workspace composer and type a single line; confirm the field starts at one line tall.
• Press Shift+Enter and confirm a newline is inserted and the input grows taller.
• Add several lines and confirm growth stops at the configured max height.
• Press Enter without Shift and confirm it submits instead of inserting a newline.
• Confirm the send button stays bottom-aligned as the composer grows.