Expose each secret status on the secrets list command

[vaurdan]

Change Summary

What and Why:
Add per-secret deployment status to fly secrets list to help users understand which secrets are deployed, staged, or partially deployed across their machines. Previously, users had no visibility into whether secrets were actually deployed to their running machines - this aligns the CLI with the existing ui-ex dashboard behavior.

Example:

$ fly secrets list -a my-app-name
NAME              DIGEST                  STATUS   
SECRET_1          522d6815e668a102        Deployed
* SECRET_2        9e2b15ab065a0dfe        Staged  

There is 1 secret not deployed. Deploy with `fly secrets deploy` to make them available.

How:

• Compare each secret's updated_at timestamp against machine release created_at timestamps to determine deployment status
• Display status column with values: Deployed, Staged, Partial, Unknown
• Add visual indicators in table output: * for staged secrets, ! for partially deployed
• Show actionable summary message when secrets need deployment (fly secrets deploy)
• Skip status computation for apps with >100 machines to avoid performance issues
• Consistent JSON output structure with/without status field

Related to:

• Requires superfly/fly-go v0.2.1 (adds UpdatedAt/CreatedAt fields to AppSecret struct)