Ignore hidden files in include paths

[oiahoon]

Summary

• ignore hidden files such as .DS_Store when recursively loading include templates
• add a CLI regression test that keeps ordinary .txt includes working

Root cause

The include glob used **/*, so Tera tried to parse hidden filesystem metadata as templates.

Validation

• reproduced the current failure with an invalid .DS_Store in the include path
• confirmed the corrected command renders both the main and included .txt template
• cargo test --locked — 24 passing
• cargo test --locked --all-features — 25 passing
• cargo check --locked
• cargo check --locked --all-features
• cargo clippy --locked --all-features --all-targets -- -D warnings
• cargo fmt --all -- --check
• git diff --check

Fixes #11