fix: return zero for top-level help flag

[wowsofine]

Summary

• Make the explicit top-level focus --help flag exit with code 0.
• Preserve the existing no-argument behavior, which still prints help and exits 1.
• Add a focused binary-level regression test for --help.

Why

The published CLI prints the top-level help text for focus --help, but exits with status 1 because the command is absent. Explicit help flags should be successful metadata commands for shell scripts, package smoke checks, and CLI conventions.

Verification

• npx --yes pnpm@10.32.1 vitest run --config config/vitest.config.ts src/bin/focus.test.ts
• npx --yes pnpm@10.32.1 lint
• npx --yes pnpm@10.32.1 typecheck
• npx --yes pnpm@10.32.1 build
• npx --yes pnpm@10.32.1 test
• FOCUS_NO_UPDATE_NOTIFY=1 CLI_VERSION=2.4.0 CORE_VERSION=1.6.0 node dist/bin/focus.js --help
• FOCUS_NO_UPDATE_NOTIFY=1 CLI_VERSION=2.4.0 CORE_VERSION=1.6.0 node dist/bin/focus.js --version
• git diff --check