Cross-link Zendesk tickets and Bugsnag errors privately

**Objective:** Cross-link Zendesk ticket `{ticket-id}` and Bugsnag error `{error-id}`, using Zendesk subdomain `{zendesk-subdomain}` to build the ticket URL.

**Step 1: Read both records**

Call `zendesk-get-ticket` with ticket-id `{ticket-id}` and `bugsnag-get-error` with error-id `{error-id}`. If either returns nothing, report which one is missing and stop.

Call `bugsnag-get-project` with `name` set to the error's project-name to get its project-id. If it returns more than one project, list them and stop.

Call `zendesk-get-ticket-comment` with ticket-id `{ticket-id}` and check whether an existing comment already contains the error's error-url. If it does, skip the Zendesk write in Step 3.

**Step 2: Confirm with the user**

Show the ticket (id, subject, requester, status) and the error (error-id, error-class, message, project-name, status), and state the two writes you are about to make:

* On Bugsnag: link the issue URL https://{zendesk-subdomain}.zendesk.com/agent/tickets/{ticket-id} to the error.
* On Zendesk: add a private internal note to the ticket containing the error's class, message, status and error-url.

Ask the user to confirm. Do not write anything until they do.

**Step 3: Write, once each**

1. Call `bugsnag-link-error-issue` once, with `error-id` `{error-id}`, the project-id from Step 1, and `issue-url` set to the URL above.
2. Call `zendesk-add-ticket-comment` once, with `ticket-id` `{ticket-id}`, `public` set to false, and a `body` like: Linked to Bugsnag error <error-class>: <message>. Status: <status>. <error-url>

**Step 4: Confirm**

Call `zendesk-get-ticket-comment` again and confirm the note is present and private. Report both writes and whether each succeeded.

**Rules & Guidelines:**

* `zendesk-get-ticket`, `zendesk-get-ticket-comment` and `zendesk-add-ticket-comment` are on the zendesk server. `bugsnag-get-error`, `bugsnag-get-project` and `bugsnag-link-error-issue` are on the bugsnag server.
* The only permitted writes are one `bugsnag-link-error-issue` call and one `zendesk-add-ticket-comment` call, after confirmation. Do NOT call `zendesk-update-ticket`, `bugsnag-create-error-issue`, `bugsnag-fix-error` or any other write tool.
* The Zendesk note must be private (`public` false). Error URLs and stack details are for agents, not the requester.

**Validation:**

* `{zendesk-subdomain}` is the part before .zendesk.com, e.g. acme. If the user passed a full URL, strip it down to the subdomain before building the link.
* If `bugsnag-link-error-issue` fails, still offer the Zendesk note, and report the Bugsnag failure plainly.
* If one write succeeds and the other fails, say exactly which, so it can be retried without duplicating the other.