Improve Copilot instructions to detect renames/moves

[pau101]

Reopened #68 on main.

In #65, Copilot incorrectly assumed WaterAnimal was removed when it was actually just moved to a different package. To reduce potential repeats of this error this PR does the following:

1. Update .github/workflows/copilot-setup-steps.yml to use git diff -M50% -l2000 to enable rename detection
   • Specifies default git 50% similarity threshold
   • Sets 2000 file rename limit, skipping detection if exceeded
2. Update .github/copilot-instructions.md to:
   • Emphasize checking diff files for renames before assuming removal
   • Show example of what renames look like in diffs
   • Reiterate that mc-source/ is not just one set of sources, but multiple, and must be searched in accordingly

This is an extension of my findings for the specific regression fix in #67

An example of this change is the WaterAnimal refactor would instead of being presented as an addition and a removal, which generally share no proximity in diffs, would now appear unified as:

diff --git a/net/minecraft/world/entity/animal/WaterAnimal.java b/net/minecraft/world/entity/animal/fish/WaterAnimal.java
similarity index 98%
rename from net/minecraft/world/entity/animal/WaterAnimal.java
rename to net/minecraft/world/entity/animal/fish/WaterAnimal.java
index e1a4dfafbc..ecff885e13 100644
--- a/net/minecraft/world/entity/animal/WaterAnimal.java
+++ b/net/minecraft/world/entity/animal/fish/WaterAnimal.java
@@ -1,4 +1,4 @@
-package net.minecraft.world.entity.animal;
+package net.minecraft.world.entity.animal.fish;
 
 import net.minecraft.core.BlockPos;
 import net.minecraft.server.level.ServerLevel;