Validate positive tokens_per_minute

[pbdeuchler]

Closes #130.

Summary:

• Reject explicit tokens_per_minute = 0 during model config validation.
• Mark tokens_per_minute with a JSON Schema minimum of 1.
• Document that tokens_per_minute must be positive when set.

Verification:

• cargo fmt --all -- --check
• cargo test -p halter-config
• cargo run -p halter-cli --bin halter-cli -- config schema
• cargo check --workspace