agent_skills: Instruct agent not to include unrelated instructions in /create-skill (#58599)

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Closes #ISSUE

Release Notes:

- N/A or Added/Fixed/Improved ...

crates/agent_skills/builtin/create-skill/SKILL.md

@@ -67,6 +67,7 @@ The body of the SKILL.md (after the frontmatter) contains the instructions the a
 3. **Include when-to-use guidance**: Help the agent understand the right context for this skill.
 4. **Reference supporting files**: Skills can include additional files in their directory. Reference them with relative paths (e.g., `templates/component.tsx`). The agent can read these files when the skill is activated.
 5. **Keep descriptions actionable**: The `description` field is the agent's primary signal for whether to load this skill. "Helps with code" is too vague. "Generate React components following the project's design system patterns" is specific.
+6. **Keep instructions focused**: Limit instructions to those relevant to the skill itself. Avoid duplicating instructions from AGENTS.md and other skills in the current conversation if they are not relevant to the skill being created
 
 ## Supporting Files