Clean up lint warnings
- Move CLI logic into src/lib.rs as `pub fn run() -> i32` so the tsk and
git-tsk binaries can each be a tiny shim under src/bin/, dropping
Cargo's "source file present in multiple build targets" warning.
- Delete unused Error::NotSelected variant.
- Delete unused properties::replace_all helper.
cargo build is now warning-free; cargo test still 61 passing.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>