fix: install.sh shows build errors instead of failing silently

cargo build --quiet swallowed compiler errors, and the subshell (...)
didn't reliably propagate exit codes across bash versions. The script
continued past a failed build, then ls/cp failed on the missing binary.

Now: removed --quiet so errors are visible, explicit exit-code check
with common-cause hints (build-essential, libc6-dev), and a secondary
check that the binary actually exists before attempting to install it.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

install.sh

@@ -31,12 +31,26 @@ echo "[ok] jq found"
 # ── Build release binary ─────────────────────────────────────────────
 echo ""
 echo "Building release binary..."
-(cd "$SCRIPT_DIR" && cargo build --release --quiet)
-echo "[ok] Built: $(ls -lh "$SCRIPT_DIR/target/release/$BINARY_NAME" | awk '{print $5}')"
+if ! (cd "$SCRIPT_DIR" && cargo build --release); then
+  echo ""
+  echo "ERROR: cargo build failed. Check the output above for details."
+  echo "  Common causes:"
+  echo "    - Missing C compiler: sudo apt install build-essential (Debian/Ubuntu)"
+  echo "    - Missing libc headers: sudo apt install libc6-dev"
+  exit 1
+fi
+
+BINARY="$SCRIPT_DIR/target/release/$BINARY_NAME"
+if [[ ! -f "$BINARY" ]]; then
+  echo "ERROR: Build succeeded but binary not found at $BINARY"
+  echo "  Check: ls $SCRIPT_DIR/target/release/"
+  exit 1
+fi
+echo "[ok] Built: $(ls -lh "$BINARY" | awk '{print $5}')"
 
 # ── Install binary ───────────────────────────────────────────────────
 mkdir -p "$INSTALL_DIR"
-cp "$SCRIPT_DIR/target/release/$BINARY_NAME" "$INSTALL_DIR/$BINARY_NAME"
+cp "$BINARY" "$INSTALL_DIR/$BINARY_NAME"
 chmod +x "$INSTALL_DIR/$BINARY_NAME"
 echo "[ok] Installed to $INSTALL_DIR/$BINARY_NAME"