feat: alias `cprr register` to `cprr add` or remove from docs

## Summary

Clarify the relationship between `cprr register` and `cprr add`.

## Motivation

In Claude project logs across multiple repos, agents invoke `cprr add` (15+ times) as the primary way to create conjectures. However, some CLAUDE.md and SKILL.md documents reference:

```bash
cprr register --id "C-NNN" --claim "description" --prediction "what would falsify this"
```

`cprr register` does not exist as a command. The actual command is `cprr add`. This causes agent confusion — they either hallucinate the `register` subcommand or fall back to `add` after a failure.

## Proposed Fix

Either:
1. **Add `register` as an alias** for `add` with `--id`, `--claim` (→ `--hypothesis`), and `--prediction` as alternate flag names
2. **Remove `register` from all documentation** and standardize on `add`

Option 1 is more forgiving of existing CLAUDE.md instructions across repos.