fix(install): support single-skill repos with SKILL.md at path root

[lgaudez]

Summary

• fetchSourceViaGit silently skipped all files when a remote repo exposes a single skill at the root of skillsPath (e.g. SKILL.md directly at path ""), because every file has relativePath.indexOf("/") === -1 and was skipped by the existing loop
• Added a fallback that groups root-level files under the requested skill name when: skillFileMap is empty after the main loop, skills filter contains exactly one non-wildcard name, and root-level files exist
• Wildcard (*) is intentionally excluded from the fallback since the skill name would be ambiguous

Note that I opened the PR because I tried to add this skill in mu rylesync.jsonc -> "https://github.com/blader/humanizer.git"

rulesync.jsonc sample :

"sources": [
  {
    "source": "https://github.com/blader/humanizer.git",
    "transport": "git",
    "ref": "8b3a17889fbf12bedae20974a3c9f9de746ed754",
    "path": "",
    "skills": ["humanizer"],
    "outputPath": ".rulesync/skills/.curated"
  },
]

Test plan

• New test: should install single-skill repo with SKILL.md at path root - verifies the happy path
• New test: should still handle classic subdirectory skill structure - regression, existing behavior unchanged
• New test: should not install root-level files when skills filter is wildcard - wildcard stays at 0 skills
• All 33 existing unit tests pass