CLI UX: 'axon run' should validate that --prompt is not empty

Problem

The `axon run` command accepts an empty string for the required `--prompt` flag, creating invalid Tasks that will fail at the API level.

Current Behavior

# Empty prompt is accepted
axon run -p "" --secret dummy --dry-run
# Output:
apiVersion: axon.io/v1alpha1
kind: Task
metadata:
  name: task-twq97
spec:
  prompt: ""  # Empty!
  ...

If submitted to the cluster (without `--dry-run`), this would fail at the Kubernetes API validation level, but only after:

1. The task resource is attempted to be created
2. Network round-trip to the API server
3. API-level validation runs

Why This is a Problem

1. Late failure: Error happens at API submission, not immediately
2. Poor error location: Users don't get helpful context about which CLI flag is the problem
3. Wasted resources: Unnecessary API call for an obvious client-side issue
4. Incomplete client validation: The CLI marks `--prompt` as required (`cmd.MarkFlagRequired("prompt")`) but doesn't validate it's non-empty

Expected Behavior

The CLI should validate that the prompt is not empty or whitespace-only:

axon run -p "" --secret dummy
Error: --prompt cannot be empty

axon run -p "   " --secret dummy
Error: --prompt cannot be empty or contain only whitespace

Impact

• Delayed error feedback: Users don't discover the issue until API submission
• Less helpful errors: API errors are less specific than CLI validation errors
• Inconsistent UX: Other validations happen client-side, but this one doesn't

Suggested Fix

Add validation in the `RunE` function after flag parsing:

prompt = strings.TrimSpace(prompt)
if prompt == "" {
    return fmt.Errorf("--prompt cannot be empty")
}

This should happen early, before any resource creation logic.

Additional Context

• The prompt field is marked as required in the API: `+kubebuilder:validation:Required`
• Found during developer experience testing
• Empty validation is a common CLI pattern (e.g., git commit requires non-empty messages)
• Related to #182 (client-side validation patterns)

Start at the axon run command's RunE function, after flag parsing and before resource creation logic. Review the existing client-side validation patterns, then run the relevant CLI tests or reproduce the empty and whitespace-only examples. Done means both inputs fail immediately with a helpful --prompt error before any API request or resource creation.