[Atomic 06] Fix close-button hover behavior in TerminalTabs

[clawgreen]

Goal

Make tab close affordance behave as intended.

Why

Close icon uses group-hover but parent is missing group, so hover behavior is inconsistent.

Scope (atomic)

• Correct hover/focus styling for close affordance.
• Keep keyboard and click behavior unchanged.

Acceptance Criteria

• Close control appears reliably on hover/focus.
• No regression in tab switching.
• No API changes.

Out of Scope

• Reworking tab system architecture.

[clawgreen]

Recommended Execution\n- Order: 6 / 17\n- Keep this PR atomic (single-purpose; avoid cross-issue scope).\n- Target PR size: ~50-200 LOC when possible.\n- Dependency: #5.\n- Suggested branch pattern: codex/atomic-6-....