streamline test coverage

[yslib]

Summary

• move reusable interpolation and platform behavior into integration tests
• cover local configuration path, relative path, and symlink semantics through the real CLI
• remove tests of error-source chains, wrapper representation, helper internals, and private execution invariants
• retain source-local unit tests only where private seams are required, such as fetch filesystem races, platform classification, Git output parsing, and diagnostic projection

Why

The suite mixed product behavior with implementation-shape and agent-observation tests. This change makes the boundary explicit: public behavior is exercised through integration tests, while source-local tests are reserved for behavior that cannot be driven through a public entry point without weakening the design.

Source-local tests decrease from 102 to 33, and the complete suite decreases from 368 to 281 tests without changing product behavior.

Validation

• cargo fmt --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace --all-features
• cargo test -p dot-cli
• cargo check -p dot-core --no-default-features --target wasm32-unknown-unknown