fix: preserve default index repo when custom index_repos configured

[Sunrisepeak]

Summary

• When users add custom index_repos in .xlings.json (e.g. adding a ros2 repo), load_index_repos_from_json_() clears and replaces globalIndexRepos_ entirely, dropping the default xim index repo
• This causes all packages in the primary index (like python, gcc, cmake, etc.) to become unfindable via xlings search or xlings install
• After loading user-defined repos, the fix checks whether the default repo is present and prepends it if missing

Reproduction

# .xlings.json has: "index_repos": [{"name": "ros2", "url": "..."}]
xlings search python    # → "no packages found"
xlings install python   # → "package not found"

Test plan

• XLINGS_HOME=/home/speak/.xlings ./build/.../xlings search python → finds xim:python
• XLINGS_HOME=/home/speak/.xlings ./build/.../xlings search ros → finds both xim:* and ros2:* packages
• Clean environment (no custom index_repos) still works as before

🤖 Generated with Claude Code