fix: log git hash on build success and startup to verify which commit is running

Writes HEAD short hash to .huskies/build_hash after successful cargo
build. Logs it on startup as [startup] Running build: <hash>. No more
guessing whether the rebuild actually deployed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/src/rebuild.rs

@@ -175,7 +175,18 @@ pub async fn rebuild_and_restart(
         return Err(format!("Build failed:\n{stderr}"));
     }
 
-    slog!("[rebuild] Build succeeded, re-execing with new binary");
+    // Write the current git HEAD to a file so we can verify which commit is running.
+    if let Ok(head) = std::process::Command::new("git")
+        .args(["rev-parse", "--short", "HEAD"])
+        .current_dir(workspace_root)
+        .output()
+    {
+        let hash = String::from_utf8_lossy(&head.stdout).trim().to_string();
+        let _ = std::fs::write(workspace_root.join(".huskies/build_hash"), &hash);
+        slog!("[rebuild] Build succeeded (commit {hash}), re-execing with new binary");
+    } else {
+        slog!("[rebuild] Build succeeded, re-execing with new binary");
+    }
 
     // 5. Send shutdown notification before replacing the process so that chat
     //    participants know the bot is going offline.  Best-effort only — we