feat: include Salesforce Apex files in default patterns

[roian6]

Summary

Salesforce Apex files are not indexed by default because .cls class files and .trigger trigger files are missing from DEFAULT_INCLUDED_PATTERNS.

This keeps the change scoped to default file matching and docs. detect_code_language() already recognizes .cls and .trigger files as apex.

Changes

• Add **/*.cls and **/*.trigger to the default include patterns.
• Document Apex in the README supported languages table.
• Add a focused regression test for the default include patterns.

Closes #175

Verification

• uv run pytest tests/test_settings.py::test_default_project_settings -q
• uv run pytest tests/test_settings.py -q
• uv run ruff check src/cocoindex_code/settings.py tests/test_settings.py
• uv run mypy src/cocoindex_code/settings.py tests/test_settings.py
• git diff --check
• uv run prek run --all-files — non-pytest hooks passed (ruff, ruff-format, uv-lock, mypy, and basic file checks); the pytest hook is blocked in this local environment by sqlite_vec/vec0.so: wrong ELF class: ELFCLASS32 and a local HuggingFace cache permission issue.

[badmonster0]

thanks a lot @roian6 , @georgeh0 can help take a look!