Claude effort controls

### Before filing

- [x] I searched [open and closed issues](https://github.com/block/berd/issues?q=is%3Aissue) for duplicates.
- [x] I am using a recent [release](https://github.com/block/berd/releases) and this doesn't already exist.
- [x] This is one request, not several bundled together.

### Closest existing issue

none found

### Is this new, or an improvement?

Improvement — Berd does this, but badly

### The problem, in your terms

Using Berd on macOS with Claude access via a Claude subscription (not a direct API key). In the model/effort picker:

Effort level (low/medium/high/xhigh/max) can only be set for Fable. No other model in the picker exposes an effort control at all.
For Fable specifically, the effort options shown stop at "high" / "max" — there's no "xhigh" option.

### What you do today

I gave up.

### What you'd like to see

Be able to choose effort levels across all Claude models.

### Why this belongs in Berd itself

No.

### Non-goals

It does not have to be one to one integration with Claude, just at least more transparency about what effort levels are available and why.

### Alternatives you considered

_No response_

### Mockups, prior art, or other context

_No response_

Start by locating the model/effort picker and the configuration that currently exposes effort levels only for Fable. Trace how Claude models and their available effort options are represented. Done means Claude models expose appropriate effort controls, including the missing xhigh option where supported, with clearer information about availability.

Written by the indexing model from the issue text.