Feature: Add color/icon support in agent front matter

Summary

Add support for color and/or icon fields in agent markdown front matter (.claude/agents/*.md) to allow visual differentiation of agents in the UI.

Use Case

When building complementary agents with different roles (e.g., executor vs verifier, or "blue team" vs "red team"), it would be helpful to visually distinguish them:

---
name: project-executor
description: Execute development subtasks
tools: Read, Write, Edit, Bash, Glob, Grep
model: haiku
color: blue   # <-- requested feature
---

---
name: project-verifier  
description: Validate completed work against requirements
tools: Read, Bash, Glob, Grep
model: sonnet
color: red    # <-- requested feature
---

Current Behavior

• The interactive /agents UI mentions "Choose a color for the subagent"
• However, this is not available as a front matter field in markdown agent definitions
• Teams sharing agents via .claude/agents/*.md cannot specify colors

Proposed Solution

Add optional front matter fields:

| Field | Type | Description |
|-------|------|-------------|
| color | string | Color name or hex code (e.g., blue, red, #3B82F6) |
| icon | string | Icon identifier (e.g., shield, check, bug) |

Benefits

1. Visual clarity - Quickly identify agent purpose in UI
2. Team conventions - Share color schemes via version control
3. Role semantics - Blue team (build) vs red team (break) patterns
4. Consistency - Same colors across team members' environments

Alternatives Considered

• Workaround: Adding [BLUE TEAM] prefix to descriptions - functional but not visually elegant
• Using the interactive UI - doesn't persist to version control for team sharing