fix(pr_files_diff): copy per-file diff bytes to break buffer aliasing

splitUnifiedDiff used bytes.Buffer to accumulate each file's diff,
then stored buf.Bytes() into the result map and called buf.Reset()
to start the next file. bytes.Buffer.Bytes() returns the buffer's
internal backing slice; Reset() resets length to 0 but reuses the
same backing array. As a result, every map entry aliased the same
storage, so all files ended up showing the LAST file's diff content.

Fix: copy the bytes into a fresh slice before storing in the map.

Adds TestPRFilesDiffPerFileIsolation as a regression test that
asserts each file entry contains its OWN diff --git header and
none of the other files' headers. Verified failing on the prior
code, passing after the fix.

Closes #25

internal/tools/pr_files_diff.go

@@ -143,7 +143,13 @@ func splitUnifiedDiff(d []byte) map[string][]byte {
 
 	flush := func() {
 		if currentFile != "" {
-			m[currentFile] = current.Bytes()
+			// Copy: bytes.Buffer.Bytes() returns the internal slice,
+			// which Reset() then reuses. Without the copy, every map
+			// entry ends up aliased to the last file's data.
+			b := current.Bytes()
+			cp := make([]byte, len(b))
+			copy(cp, b)
+			m[currentFile] = cp
 			current.Reset()
 		}
 	}