agents: document runtime version checks and multi-version pytest_test bzlmod rules (#4108)

WORKSPACE mode resolves pip dependencies for only the host Python
version, causing multi-version pytest targets to fail when executed
against non-host interpreters. Additionally, relying on static type
ignore comments in version-dependent code obscures actual runtime
compatibility requirements across supported Python releases.

To address this, agent guidelines are updated to require explicit
runtime Python version checks instead of static type ignores, and to
restrict multi-version pytest_test targets to Bzlmod mode using
SUPPORTS_BZLMOD. The allowed PR commit prefixes are also updated to
include agents: for changes to agent rules and prompts.

.agents/rules/pr.md

diff --git a/.agents/rules/pr.md b/.agents/rules/pr.md
index ae75aa3..bc6f87f 100644
--- a/.agents/rules/pr.md
+++ b/.agents/rules/pr.md
@@ -25,6 +25,8 @@
   (`.github/workflows/`).
 * `tests:`: Test-only changes and test-helper fixes (never `fix:` or
   `fix(tests):`).
+* `agents:` / `agents(<scope>):`: Agent rules, skills, and prompts
+  (`.agents/`).
 
 ## PR Commit Workflow Invariant
 * Once a Pull Request is created, always make new commits or merge commits.

.agents/rules/python.md

diff --git a/.agents/rules/python.md b/.agents/rules/python.md
index 0c4fd69..fd7aef7 100644
--- a/.agents/rules/python.md
+++ b/.agents/rules/python.md
@@ -26,6 +26,9 @@
   link to its definition in the docstring.
 
 ## Type Checking & Annotations
+* **Version differences**: Guard version-specific arguments or APIs with runtime
+  checks (e.g. `if sys.version_info >= (3, 13):`) instead of
+  `# pyrefly: ignore[...]` comments.
 * **`importlib.metadata` `PackagePath`**: `f.locate()` is typed as `PathLike`.
   Wrap with `pathlib.Path(f.locate())` to call `.exists()`, `.is_file()`, etc.
 * **In-file disables vs target skipping**: Prefer

.agents/rules/testing.md

diff --git a/.agents/rules/testing.md b/.agents/rules/testing.md
index dceb91d..cd56f69 100644
--- a/.agents/rules/testing.md
+++ b/.agents/rules/testing.md
@@ -37,3 +37,7 @@
 * **Platform Constraints**: Restrict OS-specific targets with
   `target_compatible_with` (e.g., `["@platforms//os:windows"]`) to skip on
   incompatible platforms.
+* **Multi-version `pytest_test`**: Restrict targets using `python_versions` to
+  Bzlmod with `target_compatible_with = SUPPORTS_BZLMOD`
+  (`//tests/support:support.bzl`). WORKSPACE mode resolves pip dependencies for
+  only the host Python version.

python/runfiles/runfiles.py

diff --git a/python/runfiles/runfiles.py b/python/runfiles/runfiles.py
index 0c56702..9650563 100644
--- a/python/runfiles/runfiles.py
+++ b/python/runfiles/runfiles.py
@@ -385,6 +385,7 @@
         for p in resolved.iterdir():
             yield self / p.name
 
+    # Return types and keyword arguments vary across Python versions in typeshed.
     @override
     def glob(  # pyrefly: ignore[bad-override]
         self,
@@ -407,6 +408,7 @@
         for p in it:
             yield self / p.relative_to(resolved)
 
+    # Return types and keyword arguments vary across Python versions in typeshed.
     @override
     def rglob(  # pyrefly: ignore[bad-override]
         self,