mount-tidb-cloud-fs: drop the internal codename from the skill description

skills/mount-tidb-cloud-fs/SKILL.md line 3, in the frontmatter description:

…Use when an agent needs to mount, access, or persist a Drive9-backed workspace in a sandbox, CI job, container, or local machine.

Suggested change — one word:

…Use when an agent needs to mount, access, or persist a durable workspace in a sandbox, CI job, container, or local machine.

Why this spot matters more than most

Drive9 is an internal name for the storage layer, not a product name anyone outside the team recognizes. And frontmatter description is the most exposed place it could sit — it's what an agent surfaces when listing available skills, so it's frequently the first and only sentence a user reads about this skill.

The real product name, TiDB Cloud Filesystem, is already in the same sentence, so dropping the codename costs nothing for the agent's matching or the reader's understanding.

Same class of issue as tidbcloud/ti-cli#53 (the Drive9 mentions in the CLI overview docs, which I understand is being fixed) — this one is outside the docs repo, so it probably wasn't on that list.

Drive9 appears exactly once in this repo, so this is the whole fix. Happy to send a PR if that's easier than applying it directly.

Edit the frontmatter description at line 3 of skills/mount-tidb-cloud-fs/SKILL.md. Replace the single internal codename occurrence with the suggested wording, then search the repository to confirm Drive9 no longer appears and review the resulting description.