ux: improve L1 command help text and examples

## Description

L1 commands (`l1 register-validator`, `l1 set-weight`, `l1 disable-validator`) have minimal help text that assumes deep protocol knowledge. Users unfamiliar with Warp messages have no guidance.

## Current state

```
platform l1 register-validator --help
Register a new validator on an L1 blockchain.

Usage:
platform l1 register-validator [flags]
```

No examples, no explanation of what Warp messages are or how to generate them.

## Improvements needed

1. Add `Long` descriptions with realistic examples for all L1 commands
2. Explain what `--message` (Warp message) is and how to obtain it
3. Explain what `--pop` (BLS proof of possession) is
4. Add `--validation-id` format explanation
5. Link to external docs for Warp message generation

## Severity
Medium - L1 features are hard to use without guidance

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