Document `batch`, `completion`, and `config set` in README

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Task summary

CHANGELOG.md and CLI help document batch, completion, and config set, but README.md has no usage sections for these commands. Add short sections with examples matching the style of existing suggest and history docs.

Why this is a good first issue

Pure documentation work — no TypeScript required. Easy to verify by comparing against commit-echo <command> --help output.

Suggested files or areas

• README.md
• src/index.ts — CLI help text for reference
• CHANGELOG.md — feature descriptions

Acceptance criteria

• README includes a batch usage section with at least one example
• README includes a completion usage section with at least one example
• README includes a config set usage section with at least one example
• Examples match actual CLI behavior

Extra context

Keep sections concise and consistent with the existing README tone. No need to duplicate full flag lists — link to --help where appropriate.