Allow persisting thinking effort level across sessions

Problem

The thinking effort level (Low / Medium / Max) resets to Medium every time Claude Code starts. Users who consistently prefer Max effort have to open /model and adjust it at the beginning of every session.

Proposed Solution

Add an effortLevel key to settings.json (or settings.local.json) that persists the default effort level across sessions:

{
  "effortLevel": "high"
}

Accepted values: low, medium, high (matching the internal representation).

Alternatively, a CLI flag like --effort high would also work, especially for users who want per-project defaults via shell aliases.

Motivation

For users working on complex codebases (e.g., hardware design, compiler work), Max effort is almost always the right choice. Having to re-select it every session is unnecessary friction, especially since model selection already persists.