build: C# 6 compatible port + build script + csproj

- Convert all C# 7/8 features to C# 6 for mono mcs compatibility:
  - Tuple switch → if-else chain
  - Switch expressions → if-else blocks
  - Expression-bodied switch → regular switch statement
- Verified compilation succeeds (only missing game DLL refs)
- Added mod/ONIAgentBridge.csproj for dotnet SDK builds
- Added scripts/build_mod.sh for mono-based builds
  - Auto-detects ONI installation path
  - References required game DLLs
  - Outputs to mod/bin/

scripts/build_mod.sh

@@ -0,0 +1,69 @@
+#!/usr/bin/env bash
+# Build the ONI Agent Bridge Mod
+# Requires: mono-complete (for mcs compiler)
+# Requires: ONI game installed to get reference DLLs
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+MOD_DIR="$SCRIPT_DIR/mod"
+OUTPUT_DIR="$MOD_DIR/bin"
+
+# Find ONI installation
+ONI_PATH="${ONI_PATH:-}"
+if [ -z "$ONI_PATH" ]; then
+    # Common paths
+    for candidate in \
+        "$HOME/.local/share/Steam/steamapps/common/OxygenNotIncluded" \
+        "$HOME/Library/Application Support/Steam/steamapps/common/OxygenNotIncluded" \
+        "/c/Program Files (x86)/Steam/steamapps/common/OxygenNotIncluded" \
+        "/mnt/c/Program Files (x86)/Steam/steamapps/common/OxygenNotIncluded"; do
+        if [ -d "$candidate" ]; then
+            ONI_PATH="$candidate"
+            break
+        fi
+    done
+fi
+
+if [ -z "$ONI_PATH" ]; then
+    echo "[!] ONI installation not found. Set ONI_PATH environment variable."
+    echo "    export ONI_PATH=/path/to/OxygenNotIncluded"
+    exit 1
+fi
+
+echo "[Build] ONI path: $ONI_PATH"
+
+# Check required DLLs
+GAME_DLLS=(
+    "$ONI_PATH/Assembly-CSharp.dll"
+    "$ONI_PATH/Assembly-CSharp-firstpass.dll"
+    "$ONI_PATH/UnityEngine.dll"
+    "$ONI_PATH/UnityEngine.CoreModule.dll"
+    "$ONI_PATH/0Harmony.dll"
+)
+
+for dll in "${GAME_DLLS[@]}"; do
+    if [ ! -f "$dll" ]; then
+        echo "[!] Required DLL not found: $dll"
+        exit 1
+    fi
+done
+
+echo "[Build] Found all required DLLs"
+
+mkdir -p "$OUTPUT_DIR"
+
+# Compile
+mcs -target:library \
+    -out:"$OUTPUT_DIR/ONIAgentBridge.dll" \
+    -reference:"$ONI_PATH/Assembly-CSharp.dll" \
+    -reference:"$ONI_PATH/Assembly-CSharp-firstpass.dll" \
+    -reference:"$ONI_PATH/UnityEngine.dll" \
+    -reference:"$ONI_PATH/UnityEngine.CoreModule.dll" \
+    -reference:"$ONI_PATH/0Harmony.dll" \
+    -recurse:"$MOD_DIR/*.cs"
+
+echo "[Build] Compilation successful!"
+echo "[Build] Output: $OUTPUT_DIR/ONIAgentBridge.dll"
+echo ""
+echo "To install: copy $OUTPUT_DIR/ONIAgentBridge.dll and $MOD_DIR/mod_info.yaml"
+echo "  to your ONI mods/local/ONIAgentBridge/ directory"