Add build.rs to create placeholder contributors.csv when absent

include_str! requires contributors.csv to exist at compile time.
The file is gitignored (generated by the CI release pipeline via
stackql-exec), so clippy/test CI jobs on non-main branches would fail
with a missing-file error. build.rs creates an empty placeholder if the
file is not present, keeping all non-release builds green.

https://claude.ai/code/session_01GzGtjMcwBXyVW3uKW4F2Ai

Author: claude

build.rs

@@ -0,0 +1,14 @@
+use std::fs;
+use std::path::Path;
+
+fn main() {
+    // contributors.csv is generated by the CI release pipeline (via stackql-exec)
+    // and embedded into the binary at compile time via include_str! in info.rs.
+    // For local builds and non-release CI jobs the file may not exist, so create
+    // an empty placeholder to keep the build green.
+    let csv_path = "contributors.csv";
+    if !Path::new(csv_path).exists() {
+        fs::write(csv_path, "").expect("Failed to create placeholder contributors.csv");
+    }
+    println!("cargo:rerun-if-changed=contributors.csv");
+}