Enforce third-person verb forms in command help

[dmerand]

WHY are these changes introduced?

Command help uses third-person verbs, such as “Lists” rather than “List”. Catch departures when contributors add or edit help text.

WHAT is this pull request doing?

Enable @shopify/cli/command-help-verb-form as an error for static command summaries and descriptions. The rule checks the first word, with a small allowlist for existing non-verb openings; it does not validate full prose or require missing help fields.

This PR stacks on #8863 and will be retargeted to main after #8863 merges. It reuses the command-path predicate from command-json-output without changing that rule's behavior.

No changeset is needed: this changes internal lint tooling, not CLI behavior.

How to manually test your changes?

In any command class, change the summary to start with a base-form verb, such as static summary = 'List the widgets.', and run ESLint on that file. The rule reports the first word and suggests a third-person form. Change it to Lists the widgets. and the report goes away.

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• The change is user-facing — I've identified the correct bump type (patch for bug fixes · minor for new features · major for breaking changes) and added a changeset with pnpm changeset add