agents(starlark): Prefer if/else over multi-line ternaries (#4193)

`buildifier` collapses multi-line ternary expressions onto a single
line, which violates the 80-column limit. Instruct agents to use
`if`/`else` statements when a ternary exceeds 80 columns.

.agents/rules/starlark.md

diff --git a/.agents/rules/starlark.md b/.agents/rules/starlark.md
index b04cacb..d6e322a 100644
--- a/.agents/rules/starlark.md
+++ b/.agents/rules/starlark.md
@@ -64,6 +64,8 @@
   existing files unless explicitly directed by the user.
 * **Line Length & Wrapping**: Wrap Markdown and Starlark lines to 80 columns in
   accordance with `.editorconfig`.
+  * **Long ternaries**: Use `if`/`else` statements when `a if cond else b`
+    exceeds 80 columns (`buildifier` collapses multi-line ternaries).
 
 ## Starlark Testing (`rules_testing`)
 * **`rules_testing` over `bazel_skylib`**: Always use `@rules_testing` (analysis