feat: add installable agent skill for code-mode sandbox workflows (#45)

[bwebs]

Summary

Adds an installable agent skill (lkr-code-mode) compatible with npx skills and .agents/skills/ conventions, teaching AI agents how to work with the Looker code-mode sandbox non-interactively.

Closes #45

Key Additions

1. Skill Definition (skills/lkr-code-mode/SKILL.md):
   • Focuses strictly on lkr code-mode sandbox execution (--code, --file, --var, --dev-mode).
   • Documents authentication options (--oauth-account, --force-oauth, --env-file, standard env vars).
   • Documents CLI discovery commands (lkr-dev-cli auth --help, lkr-dev-cli code-mode --help, lkr-dev-cli code-mode sandbox --help).
   • SDK discovery builtins (dir(), help(), lookup(), lookup_type(), search_with_lookups(), examples(), readme()).
   • Common SDK recipes and agent non-interactive execution guidelines.
2. Bundled Helper Scripts (skills/lkr-code-mode/scripts/):
   • personal_folder_dashboards.py: Inspect user, extract personal_folder_id, retrieve personal dashboards.
   • order_items_query.py: Run inline explore query on thelook / order_items with status filter.
   • traverse_folders.py: Recursively gather dashboards and looks from all nested subfolders.
   • readme.py & examples.py: In-sandbox discovery docs and recipes.
3. Symlink Structure:
   • .agents/skills/lkr-code-mode symlinked directly to ../../skills/lkr-code-mode.
4. Docs & Builtins Updates:
   • Top-level README.md updated with npx skills add lkrdev/cli --skill lkr-code-mode install instructions.
   • Updated lkr/codemode/readme.py and lkr/codemode/examples.py with personal folder and inline query examples.

Verification

• uv run ruff check . passed.
• uv run pytest tests/test_codemode.py passed (24/24).

[gemini-code-assist]

Code Review

This pull request introduces the lkr-code-mode agent skill, adding documentation, examples, and helper scripts to enable AI agents to execute Python code and Looker SDK operations non-interactively. The feedback suggests replacing absolute local file URLs in the documentation with relative paths, and adding defensive checks to the helper scripts to handle potential None values gracefully and prevent runtime exceptions.