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/explanation.md

@@ -0,0 +1,18 @@
+# explanation
+
+this document shall serve as the explanation for the `main.py` script. here's the inputs (on the left) and outputs of the script:
+
+```mermaid
+flowchart LR
+	f["(optional) config file path"] -->|read from passed args| b
+	a[sync.toml / sync.json] -->|read input| b[main.py]
+	c[local_folders] -->|read input| b
+	b -->|rsync folders| d[remote_folder]
+	b -->|write logs| e[folder: ~/.rsync-logs/]
+```
+
+1. If not given explicitly, read config (`_load_raw` + `load_config`) and return `cfg`
+2. Start logging
+3. Fire up the thread pool
+   1. run `sync_folder` for each `folder` in `local_folders` against target `remote_folder`
+4. Log results