feat: add `cprr update` command

## Context

When using `cprr` in the qwen3-steering project, updating a conjecture requires two separate commands:

```bash
cprr status 6 refuted
cprr evidence 6 "..."
```

An `update` subcommand would allow combining status changes with evidence and metadata updates in a single invocation.

## Proposed usage

```bash
cprr update 6 --status refuted --evidence "Single-layer wins. Score=10.91 vs multi=6.07"
cprr update 10 --status refuted --evidence "Stability=0.12, below 0.5 threshold"
cprr update 6 --title "New title" --tag new-tag
```

## Flags

- `--status ` — set status (with guard bypass like `cprr status`)
- `--evidence ` — append evidence entry
- `--title ` — update title
- `--hypothesis ` — update hypothesis
- `--tag ` — add tag
- `--remove-tag ` — remove tag

At least one flag required. All flags optional, combinable.

## Motivation

Discovered during batch CPRR updates after running experiments 05-06 in `aygp-dr/qwen3-steering`. Having to run `status` then `evidence` separately is error-prone (easy to forget the evidence step after status change).