[Atomic 12] Add TerminalGroup for collapsible command/output sections

[clawgreen]

Goal

Add a TerminalGroup component for collapsing noisy feed sections.

Why

Grouping reduces visual overload in long command streams.

Scope (atomic)

• New component with expandable/collapsible state.
• Header row (title + summary + toggle).
• Accessible button semantics.

Acceptance Criteria

• Collapsed state hides child content.
• Expanded state is default-configurable.
• ARIA attributes are present and correct.

[clawgreen]

Recommended Execution\n- Order: 12 / 17\n- Keep this PR atomic (single-purpose; avoid cross-issue scope).\n- Target PR size: ~50-200 LOC when possible.\n- Dependency: #11.\n- Suggested branch pattern: codex/atomic-12-....