fix: restore persisted local mode settings

[Cruel]

Restores persisted local/cloud mode selection after the Rust migration.

The previous TypeScript CLI supported steel settings and persisted the selected mode in config.json via the existing instance field. During the Rust migration, the settings command implementation and config field were ported, but the command was no longer registered and instance was not used during API mode resolution.

This PR:

• restores the steel settings command
• honors instance: "local" when resolving API mode
• preserves existing --local and --api-url behavior
• documents the supported config.json values
• adds regression coverage for both mode resolution and command registration

Local endpoint precedence is otherwise unchanged.