Add local config override support and full environment hierarchy to EnvironmentConfiguration

[Copilot]

EnvironmentConfiguration had environment detection and single-file resolution but was missing the third tier of the config hierarchy (config.local.json) and a way to retrieve the full ordered file list for layered loading.

Changes

EnvironmentConfiguration / IConfigurationManager

• GetLocalOverrideFilePath(baseFilePath) — derives the .local.json path (e.g. config.local.json) unconditionally; callers decide whether to act on its existence
• GetConfigurationFiles(baseFilePath) — returns an ordered IReadOnlyList<string> of existing files representing the full hierarchy (base → env-specific → local); uses Path.GetFullPath + case-insensitive comparison to prevent duplicates when paths alias the same file

Test coverage
Added 14 tests to ConfigurationEnhancementsTests:

• Explicit detection of all four environments (Development, Testing, Staging, Production) via ACAT_ENVIRONMENT
• Case-insensitive env var parsing
• Fallback chain: ACAT_ENVIRONMENT → DOTNET_ENVIRONMENT → ASPNETCORE_ENVIRONMENT
• GetLocalOverrideFilePath (valid path, null input)
• GetConfigurationFiles across all hierarchy combinations (base-only, base+env, all three layers, base+local)

Housekeeping

• .gitignore — added *.local.json so developer override files are never accidentally committed
• README.md / CONFIGURATION_ENHANCEMENT_GUIDE.md — documented the four-level hierarchy and local override workflow

Usage

var envConfig = new EnvironmentConfiguration(logger);
envConfig.SetEnvironment(ConfigurationEnvironment.Development);

// Returns e.g. ["config.json", "config.Development.json", "config.local.json"]
// (only files that exist on disk, in merge order)
IReadOnlyList<string> files = envConfig.GetConfigurationFiles("config.json");

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Environment-Specific Configuration</issue_title>
<issue_description>Support development/production/test configuration files with layering.

Acceptance Criteria:

• Environment detection (dev/prod/test)
• Base + override config file loading
• Environment variable override support
• Documentation for environment setup
• Tests for all environments

Configuration Hierarchy:

1. Base configuration (config.json)
2. Environment configuration (config.dev.json)
3. Local overrides (config.local.json - gitignored)
4. Environment variables (ACAT_* prefix)

Files to Create:

• Libraries/ACATCore/Utility/EnvironmentConfiguration.cs
• Environment detection logic
• Config file resolution logic

---

Epic: Configuration System Enhancement

Estimated Effort: 2 days

---

Parent Feature: #191 - Configuration System Enhancement

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Environment-Specific Configuration #220

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.