feat: shimmer + spinner CLI loading UX for agent responses

[qhkm]

Improve the CLI UX when waiting for AI responses and during multi-step tool execution.

Current state: bare [tool_name] Running... done with no thinking indicator.

Planned UX:

• Gradient text shimmer on 'Thinking...' (color wave sweeps across letters)
• Spinner animation alongside shimmer
• Step numbering with tool argument context
• Checkmarks for completed steps
• Separator line before final response
• Shimmer restarts between tool batches

Ref: Option A from brainstorm (gradient text shimmer).