Add help case agent skill: file a case, add a comment with a working link - #26

What this PR adds or changes

One agent skill, help-case-agent-skill: route an HR question to the right help case type, prepare the case for
approval, and prepare a comment on an existing case.

It also documents something I could not find written down: how to put a working link in a help case comment through
the REST API. A question on the developer forum asked for exactly this, so the skill carries the tested request shape.

Checklist for new or changed examples

• The example lives entirely in its own folder under examples/
• node scripts/validate-examples.mjs --check passes (valid example.json, README present, index table in sync)
• The README says what the artifact is, how to use it, and what to change before deploying it (the "Before you deploy" section)
• No credentials, tenant names, or real personal data anywhere in the folder
• node scripts/audit-examples.mjs --changed passes, or the findings it reports are explained below

Anything reviewers should know?

• The audit was run with --skip-arcane (hub rules only); the folder holds markdown and JSON, no Extend source.
• Every operation in SKILL.md was run on a development tenant on 11 November 2026 with helpCase/v4, and each comment
  was checked on the case page. What that showed:
  • comment.textBody is plain text. An address or an <a> tag in it is shown as typed and is not clickable.
  • comment.content takes a rich text document as a string. A link mark on a text node is drawn as a link, and it is
    the same shape the case page's own editor writes when an address is typed.
  • The page builds the link from the node's text, not from href, so the link text has to be the address itself.
  • Sending textBody and content together is refused with a 400.
• This is behaviour observed on one tenant, not taken from documentation, and the README says so. If the team knows of
  a documented rule for link labels, I will gladly correct the skill.
• Kept separate from Add four agent skills: expense report, time tracking, worker data quality, learning enrolment #25 so each can be reviewed on its own.

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