Make slash commands bold in welcome message for better visibility

Problem

The current welcome message displays all text (including slash commands) in dimmed styling,
making the available commands less prominent. Users need to quickly identify the available
commands (/init, /status, /approvals, /model) when starting Codex.

### Proposed Solution
Make the slash command text bold and white while keeping their descriptions dimmed for visual
contrast. This would improve the visual hierarchy and make commands more discoverable.

### Current Behavior
To get started, describe a task or try one of these commands:

/init - create an AGENTS.md file with instructions for Codex
/status - show current session configuration and token usage
/approvals - choose what Codex can do without approval
/model - choose what model and reasoning effort to use
(All text appears dimmed)

### Expected Behavior
The command names (/init, /status, /approvals, /model) should appear bold and white,
while the descriptions remain dimmed for contrast.

### Impact

• Low risk - UI-only change, no functional changes
• High visibility - Affects every user's first interaction with Codex
• Improves UX - Better command discoverability

### Implementation Notes
This would involve modifying the new_session_info function in tui/src/history_cell.rs to use
separate Span styling for commands vs descriptions.