feat: add remote configuration sources

[yslib]

Summary

• allow --config SOURCE to load local paths or HTTPS manifests
• add --git REPOSITORY --git-worktree PATH with clone-or-reuse semantics and strict repository-local origin validation
• preserve native non-UTF-8 configuration paths and document all source/path-context behavior

Design boundaries

• source acquisition remains entirely in dot-cli; dot-core::ConfigFile is unchanged
• HTTPS acquisition is in-memory and independent from dot-core Fetch Content
• Git is invoked externally and existing worktrees are never updated or repaired

Validation

• cargo fmt --all -- --check
• cargo clippy --locked --workspace --all-targets --all-features -- -D warnings
• cargo test --locked --workspace --all-targets --all-features
• cargo test --locked -p dot-cli --all-targets
• native and wasm no-default dot-core checks
• release dot-cli build