feat: select backup sources by marker file

[magrathean-uk]

Summary

Adds --include-if-present to select local directory trees containing a marker filename.

Validation

• 3 marker selection/rejection unit tests
• Configuration reference and full-profile example
• cargo clippy --locked --all-targets --features release -- -D warnings
• cargo fmt --all -- --check
• git diff --check
• typos

Fixes #1382.