feat(tui): collapse working blocks by default with a global /working toggle

[mdev34-lab]

Summary

Working blocks should be collapsed by default, while remaining globally expandable through a /working command, analogous to how /thinking controls visibility of thinking/CoT.

Expected behavior

• Working blocks render collapsed by default.
• /working toggles the global visibility/expanded state of working blocks.
• The behavior should be consistent across working blocks in the conversation, rather than requiring each block to be opened individually.
• /working should follow the existing interaction model of /thinking as closely as practical.

Motivation

Working traces can become very large and currently consume substantial vertical space. The default should keep the useful final output prominent while still allowing users to inspect the full working trace when desired.

Acceptance criteria

• Newly rendered working blocks are collapsed by default.
• /working globally toggles working-block visibility/expansion.
• Existing working blocks respond to the global toggle.
• The toggle does not affect ordinary assistant output or thinking blocks.
• Add regression coverage for the default state and command toggle.