Rename "Prompt Library" to "Rules Library" (#29349)

There's probably more to do to fully make the transition, and we'll
still debate a bit internally whether this is the name, but just opening
this PR up now for visibility.

Release Notes:

- N/A

crates/agent/Cargo.toml

@@ -62,7 +62,7 @@ parking_lot.workspace = true
 paths.workspace = true
 picker.workspace = true
 project.workspace = true
-prompt_library.workspace = true
+rules_library.workspace = true
 prompt_store.workspace = true
 proto.workspace = true
 release_channel.workspace = true