DOC: Cross-reference ruff/black/isort version pins between the two files

ruff, black and isort are each pinned twice: once in pyproject.toml's
testing extra (the range a developer installs from) and once in
.pre-commit-config.yaml (the exact rev pre-commit builds its
environment from). Only black and isort carried a one-sided
"keep synced" comment, in .pre-commit-config.yaml only; ruff had
none anywhere. A contributor editing one pin had no in-file pointer
to the other.

Adds a same-line comment at each of the six pin sites naming the
file holding its counterpart, so following either pin leads
directly to the other. No version or behavior change.

Author: hjmjohnson

.pre-commit-config.yaml

@@ -30,7 +30,7 @@ repos:
       - id: mixed-line-ending
 
   - repo: https://github.com/astral-sh/ruff-pre-commit
-    rev: v0.16.5
+    rev: v0.16.5  # ruff version, keep synced with the ruff pin in pyproject.toml's testing extra
     hooks:
     -   id: ruff-check
         args: ["--fix"]
@@ -41,7 +41,7 @@ repos:
           )
 
   - repo: https://github.com/psf/black-pre-commit-mirror
-    rev: 26.5.1  # Black version, keep synced with MONAI requirements
+    rev: 26.5.1  # Black version, keep synced with the black pin in pyproject.toml's testing extra
     hooks:
       - id: black
         language_version: python3
@@ -53,7 +53,7 @@ repos:
           )
 
   - repo: https://github.com/pycqa/isort
-    rev: 9.0.1  # isort version, keep synced with MONAI requirements
+    rev: 9.0.1  # isort version, keep synced with the isort pin in pyproject.toml's testing extra
     hooks:
       - id: isort
         name: isort (python)

pyproject.toml

@@ -160,9 +160,9 @@ transformers = ["transformers>=5.5.0"]  # 5.x needs the transchex BertLayer/Bert
 zarr = ["zarr"]
 # these dependencies are for testing/building only, they aren't needed for regular use so don't appear in "all"
 testing = [
-  "black>=26.5.1",
+  "black>=26.5.1",  # keep in sync with the black rev in .pre-commit-config.yaml
   "coverage>=5.5",
-  "isort>9.0.0",
+  "isort>9.0.0",  # keep in sync with the isort rev in .pre-commit-config.yaml
   "mccabe",
   "packaging",
   "parameterized",
@@ -171,7 +171,7 @@ testing = [
   "pycodestyle",
   "pyflakes",
   "pyrefly>=1.0.0",
-  "ruff>=0.16.5",
+  "ruff>=0.16.5",  # keep in sync with the ruff rev in .pre-commit-config.yaml
   "tomli",  # used in print_dependencies.py for Python<3.11
   "types-PyYAML",
   "types-setuptools"