[FEATURE] Customizable icons for @ mention suggestion items

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

The @ mention suggestion UI displays hardcoded prefix icons for each item type:

• + for files / directories
• * for agents (subagents)
• ◇ for MCP resources

These icons cannot be changed, making it difficult to visually distinguish item types at a glance — especially in terminals with limited color support, or when using custom themes.

Proposed Solution

Allow users to configure the icons per item type via settings.json or a similar config mechanism. For example:

  {
    "mentionIcons": {
      "file": "+",
      "agent": "🤖",
      "mcp": "◇"
    }
  }

Alternatively, allow agent definitions (.claude/agents/*.md) to specify their own icon in the frontmatter:

  ---
  name: react-best-practices-reviewer
  icon: "⚛️"
  ---

Alternative Solutions

No workarounds currently exist. The icons appear to be hardcoded in the CLI's TUI rendering layer.

Priority

Low - Nice to have

Feature Category

Interactive mode (TUI)

Use Case Example

1. I have multiple subagents defined in .claude/agents/
2. When typing @react- in the prompt, the suggestion list shows several items all prefixed with *
3. With custom icons, each agent could have a distinct icon, making the list scannable without reading the full name

Additional Context

_No response_