Add loading spinner/animation while agent executes

[initcron]

Problem

When running aofctl run agent, there's a noticeable delay between pressing enter and seeing output. Users have no visual feedback that the agent is working.

Proposed Solution

Add a loading spinner/animation using indicatif crate that shows while the agent is executing.

Requirements:

• Show spinner immediately after command starts
• Hide spinner when output is ready
• Cross-platform support
• Clean integration with existing output

Location: crates/aofctl/src/commands/run.rs

Related to: UX improvements from getting-started tutorial feedback

Acceptance Criteria

• Spinner shows during agent execution
• Spinner clears when output appears
• Works on macOS, Linux, Windows
• No interference with actual output

[initcron]

✅ Implemented! Loading spinner now shows while agent executes.

Features:

• Animated spinner with clean updates every 100ms
• Shows "Executing agent: " message
• Automatically clears when execution completes
• Works seamlessly with all output formats

Demo:

aofctl run agent docker-health.yaml --input "show me stats"

Shows spinning animation until output is ready!