agents(rules): add Starlark dict union and Pyrefly PackagePath conventions (#4038)

Starlark supports the dictionary union operator (|), making skylib's
dicts.add redundant. When using importlib.metadata in Python,
f.locate() is typed as PathLike and lacks filesystem methods like
.exists() and .is_file() under Pyrefly unless wrapped with
pathlib.Path.

Add rules instructing agents to prefer the dict union operator (|)
over dicts.add in Starlark and to wrap f.locate() calls with
pathlib.Path() in Python type-annotated code.

.agents/rules/python.md

diff --git a/.agents/rules/python.md b/.agents/rules/python.md
index 21f6671..701397f 100644
--- a/.agents/rules/python.md
+++ b/.agents/rules/python.md
@@ -14,6 +14,8 @@
   link to its definition in the docstring.
 
 ## Type Checking & Annotations
+* **`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 `# pyrefly: ignore[<error-code>]`
   (e.g. `[missing-import]`) over `tags = ["no-pyrefly"]`.
 * **No blanket ignores**: NEVER use bare `# type: ignore` or literal

.agents/rules/starlark.md

diff --git a/.agents/rules/starlark.md b/.agents/rules/starlark.md
index 4517b06..2eccba7 100644
--- a/.agents/rules/starlark.md
+++ b/.agents/rules/starlark.md
@@ -41,6 +41,8 @@
   `while` loops; iterate over fixed-size ranges or explicit collections.
 
 ## Code Style & Conventions
+* **Dict union (`|`)**: Use `|` instead of `dicts.add(...)` from
+  `@bazel_skylib//lib:dicts.bzl` when merging dictionaries.
 * **Docstring Formatting Invariants**: Use triple-quoted strings for multi-line
   docstrings without trailing backslashes (`\`) for line continuation.
 * **No Bazel Copyright Headers**: Do not add Bazel copyright headers to new or