Add `config set` example to root CLI help text

[404-Page-Found]

Task summary

Root --help lists examples for init, config, suggest, history, batch, and completion, but not config set <key> <value>. Adding one example line improves discoverability of a feature that exists but is easy to miss.

Why this is a good first issue

Single-string addition in the CLI entry point. Immediately verifiable with node dist/index.js --help.

Suggested files or areas

• src/index.ts — root help examples section
• tests/cli-help.test.mjs — optionally extend assertion

Acceptance criteria

• Root --help includes a config set example (e.g. commit-echo config set model gpt-4.1)
• Example uses a realistic key/value pair
• npm test passes

Extra context

Keep the example consistent with other help examples in style and length.