ux: add confirmation prompts for irreversible operations

## Description

Only `keys delete` has a confirmation prompt. Other high-stakes operations proceed without confirmation.

## Operations missing confirmation

1. **`subnet convert-l1`** - irreversible conversion, should always confirm
2. **`validator add`** - locks significant funds for a minimum duration
3. **Large transfers** - transfers above a configurable threshold

## Current behavior

```bash
# This irreversible operation runs immediately with no confirmation
platform subnet convert-l1 --subnet-id ... --chain-id ... --mock-validator
```

## Expected behavior

```
Converting subnet 2QYfFcfZ9... to L1. This operation is IRREVERSIBLE.
Type 'yes' to confirm:
```

## Severity
Medium - safety improvement for production use

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