chore(env): remove deprecated .env.custom and add 90-project.env

Author: mrz1836

.github/.env.custom

@@ -0,0 +1,33 @@
+# ================================================================================================
+# 🏗️ PROJECT-SPECIFIC CONFIGURATION (90-project.env)
+# ================================================================================================
+#
+# Purpose: Project-specific overrides for postmark
+# Load Order: 90 (loaded near-last, overrides all default and tool-specific settings)
+#
+# This file contains configuration values unique to this project that override
+# defaults defined in earlier env files (00-core, 10-*, 20-*).
+#
+# Override in: 99-local.env (local development only, not committed)
+#
+# ================================================================================================
+
+# ================================================================================================
+# 🎯 GO VERSION CONFIGURATION
+# ================================================================================================
+
+# Secondary Go version for compatibility testing
+GO_SECONDARY_VERSION=1.18.x
+
+# ================================================================================================
+# 📊 COVERAGE CONFIGURATION
+# ================================================================================================
+
+# Coverage Report Provider (codecov for this project)
+GO_COVERAGE_PROVIDER=codecov
+
+# Codecov Configuration
+CODECOV_TOKEN_REQUIRED=true
+
+# Coverage Exclusions (aligned with codecov.yml ignore paths)
+GO_COVERAGE_EXCLUDE_PATHS=test/,vendor/,testdata/,.github/,.mage-cache/,.vscode/,bin/,example/,examples/,mocks/,testing/,docs/