Set up Copilot coding agent instructions and environment pre-configuration

.github/copilot-instructions.md

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+# Copilot Instructions for TableTool
+
+## Project Overview
+
+TableTool is a macOS CSV editor written in Objective-C. It automatically detects CSV format specifications (delimiter, encoding, decimal separator, quoting style) and provides a grid-based UI for editing CSV files.
+
+## Repository Structure
+
+- **`Table Tool/`** – Main application source (Objective-C `.h`/`.m` files)
+  - `CSVReader.h/.m` – Reads and parses CSV data
+  - `CSVWriter.h/.m` – Writes CSV data
+  - `CSVHeuristic.h/.m` – Auto-detects CSV format from file content
+  - `CSVConfiguration.h/.m` – Data model for CSV format settings (delimiter, encoding, quote character, etc.)
+  - `Document.h/.m` – NSDocument subclass managing the open file
+  - `TTFormatViewController.h/.m` – UI for selecting/overriding CSV format
+  - `TTPreferencesWindowController.h/.m` – Preferences window
+- **`Table ToolTests/`** – XCTest unit tests (Objective-C)
+- **`Table Tool.xcodeproj`** – Xcode project file
+
+## Build & Test Commands
+
+Build the project:
+```bash
+xcodebuild build \
+  -project "Table Tool.xcodeproj" \
+  -scheme "Table Tool" \
+  -destination "platform=macOS" \
+  -configuration Debug \
+  CODE_SIGN_IDENTITY="" CODE_SIGNING_REQUIRED=NO CODE_SIGNING_ALLOWED=NO
+```
+
+Run all tests:
+```bash
+xcodebuild test \
+  -project "Table Tool.xcodeproj" \
+  -scheme "Table Tool" \
+  -destination "platform=macOS" \
+  -configuration Debug \
+  ONLY_ACTIVE_ARCH=NO \
+  MACOSX_DEPLOYMENT_TARGET=10.13 \
+  CODE_SIGN_IDENTITY="" \
+  CODE_SIGNING_REQUIRED=NO \
+  CODE_SIGNING_ALLOWED=NO \
+  ENABLE_TESTABILITY=YES
+```
+
+## Coding Conventions
+
+- **Language**: Objective-C for all source files. The project has a Swift bridging header (`Table Tool-Bridging-Header.h`) but no Swift source files yet.
+- **Naming**: Follow Apple/Cocoa conventions — `camelCase` for methods and variables, `PascalCase` for classes and types. Prefix custom classes with `TT` (e.g., `TTFormatViewController`) or `CSV` (e.g., `CSVReader`).
+- **Memory management**: Use ARC (Automatic Reference Counting).
+- **Error handling**: Pass `NSError **` out-parameters for recoverable errors (see `CSVReader -readLineWithError:`).
+- **Tests**: Add XCTest test methods in `Table ToolTests/Table_ToolTests.m`. Use `XCTAssert*` macros. Add any required test CSV files under `Table ToolTests/Reading Test Documents/` or `Table ToolTests/Heuristic Test Documents/`.
+
+## Key Notes
+
+- The macOS deployment target in the Xcode project is **10.8**. CI builds override it to **10.13** via the `MACOSX_DEPLOYMENT_TARGET` build flag.
+- Do not add third-party dependencies unless absolutely necessary; the project has no external package manager (no CocoaPods, Carthage, or SPM).
+- All UI is AppKit-based (no SwiftUI).
+- CI runs on `macos-14` via GitHub Actions (see `.github/workflows/ci.yml`).

.github/copilot-setup-steps.yml

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+steps:
+  - name: Show Xcode version
+    run: xcodebuild -version
+
+  - name: Build project
+    run: |
+      set -o pipefail
+      xcodebuild build \
+        -project "Table Tool.xcodeproj" \
+        -scheme "Table Tool" \
+        -destination "platform=macOS" \
+        -configuration Debug \
+        ONLY_ACTIVE_ARCH=NO \
+        MACOSX_DEPLOYMENT_TARGET=10.13 \
+        CODE_SIGN_IDENTITY="" \
+        CODE_SIGNING_REQUIRED=NO \
+        CODE_SIGNING_ALLOWED=NO \
+        ENABLE_TESTABILITY=YES