add project tooling and test suite for many-rsync

- Add pyproject.toml: hatchling build, many-rsync entrypoint, ruff/mypy/pytest config
with 60% coverage floor
- Add uv.lock for reproducible dev installs
- Add .pre-commit-config.yaml: ruff (with --fix) + mypy hooks
- Add test_main.py: unit tests for _build_rsync_cmd, _load_raw, and load_config
covering happy paths and FATAL exit cases
- Add explanation.md: architecture overview with flowchart
- main.py: refactor into typed, testable functions (_load_raw, _build_rsync_cmd
extracted); add RsyncParameters/Config TypedDicts; add rsync_parameters config support
(rsync_path, exclude_from); harden validation (n clamped, log_level validated)
- README.md: update install instructions and document all config fields including
rsync_parameters

many-rsync/README.md

@@ -4,6 +4,20 @@
 
 rsync a set of folders to the remote in parallel.
 
+### how to use it
+
+Install to ~/.local/bin:
+
+```sh
+uv tool install .
+```
+
+After code changes, reinstall:
+
+```sh
+uv tool install --force .
+```
+
 ### config
 
 use TOML (preferred) or JSON.