fix(git-transport): support path: "." / "" so skills at the repo root are fully fetched

[besdar]

rulesync install with transport: "git" only fetches root-level files when the configured path points at the repository root ("", ".", or "./"). For example, given:

{
  "source": "https://github.com/feature-sliced/skills",
  "path": ".",
  "skills": ["feature-sliced-design"],
  "transport": "git"
}

…only README.md ends up in .rulesync/skills/.curated/feature-sliced-design/, while the actual skill (feature-sliced-design/SKILL.md + the references/ folder) is silently dropped. No error is reported, which makes the failure mode quite confusing — install happily says “Fetched 1 skill(s)”.

Root cause

fetchSkillFiles in src/lib/git-client.ts runs:

git clone --depth 1 --branch <ref> --no-checkout --filter=blob:none -- <url> <tmpDir>
git -C <tmpDir> sparse-checkout set -- <skillsPath>
git -C <tmpDir> checkout

git sparse-checkout in cone mode (the default since git 2.27) treats . / "" / ./ as “top-level files only — exclude every subdirectory”. So after checkout, the working tree contains only the files directly at the repo root, and the subsequent walkDirectory(<tmpDir>/.) only finds those root files.

The downstream groupRemoteFilesBySkillRoot fallback at src/cli/commands/install/install-rulesync.ts (the “rootLevelFiles → singleSkillName” branch) then groups every root-level file under the requested skill name. That’s why the README is written as if it were the body of the skill.

This makes it impossible to install via transport: "git" from any repository that stores skills at the root (<repo>/<skill-name>/SKILL.md without a skills/ container directory). feature-sliced/skills is one such repository — the github transport works fine because it uses the REST listDirectory API which has no equivalent quirk.

Fix

In fetchSkillFiles, treat "", ".", and "./" as “the whole repo”:

• Call git -C <tmpDir> sparse-checkout disable instead of sparse-checkout set -- ., so the full working tree is restored.
• Use <tmpDir> itself as the skills root passed to walkDirectory, instead of <tmpDir>/..

For any other path value the behavior is unchanged.

Repro

Before the fix:

$ cat rulesync.jsonc
{
  "sources": [
    {
      "source": "https://github.com/feature-sliced/skills",
      "path": ".",
      "skills": ["feature-sliced-design"],
      "transport": "git"
    }
  ]
}

$ rulesync install
Installing skills from 1 source(s)...
Fetched 1 skill(s) from https://github.com/feature-sliced/skills: feature-sliced-design
Installed 1 skill(s) from 1 source(s).

$ ls .rulesync/skills/.curated/feature-sliced-design
README.md          # ← only this file, no SKILL.md, no references/

After the fix (same config):

$ rulesync install
Installing skills from 1 source(s)...
Fetched 1 skill(s) from https://github.com/feature-sliced/skills: feature-sliced-design
Installed 1 skill(s) from 1 source(s).

$ ls .rulesync/skills/.curated/feature-sliced-design
SKILL.md  references/

$ ls .rulesync/skills/.curated/feature-sliced-design/references
asset-handling.md       excessive-entities.md      layer-structure.md
cross-import-patterns.md framework-integration.md   migration-guide.md
practical-examples.md

Changes

• src/lib/git-client.ts — branch on skillsPath ∈ {"", ".", "./"}: call sparse-checkout disable and use the clone root directly as the skills root.
• src/lib/git-client.test.ts — added two test cases:
  1. it.each(["", ".", "./"])("disables sparse-checkout when skillsPath is %j (repo root)", …) — asserts sparse-checkout disable is invoked and sparse-checkout set is not.
  2. "uses the clone directory itself as the skills root when skillsPath is the repo root" — asserts files are read from <tmpDir> directly.

Notes

• The fallback path for repos like <repo>/SKILL.md (a single root-level skill, no per-skill subdirectory) already exists in groupRemoteFilesBySkillRoot and continues to work; this PR only makes the data reach that fallback when using transport: "git".
• The github transport (transport: "github", the default) is unaffected — it never had this issue because it lists directories via the REST API.