Fix git-repo skill frontmatter to comply with the skills-ref validation spec

[DrumRobot]

Problem

Running the skills-ref validator against the git-repo skill's frontmatter reports two failures:

1. The frontmatter includes a depends-on field, which is not in the validator's allowed field list (allowed-tools / compatibility / description / license / metadata / name).
2. The description field is roughly 1636 characters, exceeding the validator's 1024-character limit.

Neither issue affects the skill's runtime behavior today, but it means the skill does not pass spec validation, and the over-length description is a maintenance smell (a frontmatter field is not the right place for that much text).

Scope

1. Shorten description to 1024 characters or fewer using standard reduction strategies (trim redundant phrasing, move detail into the skill body instead of the frontmatter field).
2. Remove the depends-on field from frontmatter, since it is not part of the allowed field list. If the dependency information it conveyed is still useful, express it in prose instead — for example as a "Topic Dependencies" section in the skill body — rather than as a frontmatter field.
3. Re-run the skills-ref validator against the skill afterward to confirm both failures are resolved.

Verification

Feature	Procedure	Expected Result
Description length	Run the skills-ref validator against the skill's frontmatter	description passes the length check (≤1024 chars)
Allowed-fields check	Run the skills-ref validator against the skill's frontmatter	No disallowed fields (e.g. depends-on) remain in frontmatter
Dependency info preserved	Review the skill body	If dependency information existed in the removed field, it is still findable in the skill body (e.g. a Topic Dependencies section)
No behavior regression	Exercise the skill's existing topics	Skill continues to load and function as before