ux: standardize success output format across commands

## Description

Success output varies inconsistently across commands:

- Keys: `"Key imported successfully!"` with structured output
- Subnet create: `"Subnet created successfully!"` + `"Subnet ID: ..."`
- Validator add: Just `"TX ID: ..."` (no success message)
- Transfer: `"Transfer complete!"`

## Suggested standard format

```
[Action] successful!
[Structured details...]
TX ID: ...
```

## Files affected
All `cmd/*.go` files

## Severity
Low - cosmetic consistency

## Source
CLI UX audit (2026-02-09)