Rename claude-academy-guide skill to academy-guide and shorten its description (#1605)

Two changes to make the skill packageable as an uploaded custom skill,
with no other content changes:

- Rename: the skill folder, the SKILL.md frontmatter name, and the
  marketplace.json entry name and path. Skill names cannot contain the
  reserved words "claude" or "anthropic" (per the agent skills
  best-practices documentation).
- Description: shortened from 1,176 to 992 characters to fit the
  1,024-character limit that skill upload validation applies to the
  SKILL.md frontmatter description. The shortened text is the version
  tested internally.

.claude-plugin/marketplace.json

@@ -52,12 +52,12 @@
       ]
     },
     {
-      "name": "claude-academy-guide",
+      "name": "academy-guide",
       "description": "Recommends relevant Claude Academy courses, tutorials, and use cases when users ask how to use Claude",
       "source": "./",
       "strict": false,
       "skills": [
-        "./skills/claude-academy-guide"
+        "./skills/academy-guide"
       ]
     },
     {