Show custom agent name in header when using --agent flag

Feature Request

When launching Claude Code with --agent <name> (or --agents with custom definitions), the header still displays "Claude Code" instead of the agent name.

Use Case

Users who define multiple personality profiles (e.g., different system prompts, permission modes, and behavioral settings) via --agents and --agent flags have no visual indicator of which profile is active during a session.

Current Behavior

• Header always shows "Claude Code" regardless of --agent value
• Terminal title is overridden by Claude Code on launch
• No persistent visual indicator of the active agent

Proposed Behavior

• Display the agent name in the header (e.g., "Claude Code // ARES" or just "ARES")
• Optionally allow agents to define a color or icon
• Reflect agent name in terminal title so tabs show it

Example Setup

claude --agents '{"ares": {"description": "Full autonomy mode", "prompt": "..."}}' --agent ares

Expected: Header shows "ARES" or "Claude Code · ARES"
Actual: Header shows "Claude Code"

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