Make session history compaction asynchronous

[Copilot]

Compaction was running synchronously in the REPL main thread, blocking user input for 1-5 seconds. Per ADR-009, this should match Python ADK's async behavior.

Changes

• Wrapped compaction logic in go func() { ... }() to run asynchronously
• Removed spinner UI (unsafe across goroutines)
• Replaced fmt.Printf with log.Printf for background logging

Before/After

Before:

// Blocks REPL for 1-5 seconds
if !hasError && r.config.SessionManager != nil {
    compactionSpinner := display.NewSpinner(...)
    compactionSpinner.Start()
    coordinator.RunCompaction(ctx, sess)
    compactionSpinner.Stop()
}

After:

// Non-blocking async execution
if !hasError && r.config.SessionManager != nil {
    go func() {
        coordinator.RunCompaction(ctx, sess)
        log.Printf("Compaction complete")
    }()
}

Follows ADR-009 reference implementation pattern (lines 847-870).

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