feat(config): add Env field to RuntimeConfig and AgentPresetInfo (#860)

Add support for agent presets to specify environment variables that
get exported when starting sessions. This enables agents to use
environment-based configuration.

Changes:
- Add Env field to RuntimeConfig struct in types.go
- Add Env field to AgentPresetInfo struct in agents.go
- Update RuntimeConfigFromPreset to copy Env from preset
- Update fillRuntimeDefaults to preserve Env field
- Merge agent Env vars in BuildStartupCommand functions
- Add comprehensive tests for Env preservation and copy semantics

This is a prerequisite for the OpenCode agent preset which uses
OPENCODE_PERMISSION='{"*":"allow"}' for auto-approve mode.

Co-authored-by: Avyukth <subhrajit.makur@hotmail.com>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

internal/config/loader_test.go

@@ -2777,3 +2777,97 @@ func TestBuildStartupCommandWithAgentOverride_NoGTAgentWhenNoOverride(t *testing
 		t.Errorf("expected no GT_AGENT in command when no override, got: %q", cmd)
 	}
 }
+
+func TestFillRuntimeDefaultsPreservesEnv(t *testing.T) {
+	t.Parallel()
+	tests := []struct {
+		name     string
+		input    *RuntimeConfig
+		wantEnv  map[string]string
+		wantNil  bool
+	}{
+		{
+			name:    "nil input returns default",
+			input:   nil,
+			wantNil: false,
+		},
+		{
+			name: "preserves Env map",
+			input: &RuntimeConfig{
+				Command: "test-cmd",
+				Env: map[string]string{
+					"TEST_VAR": "test-value",
+					"JSON_VAR": `{"*":"allow"}`,
+				},
+			},
+			wantEnv: map[string]string{
+				"TEST_VAR": "test-value",
+				"JSON_VAR": `{"*":"allow"}`,
+			},
+		},
+		{
+			name: "nil Env stays nil",
+			input: &RuntimeConfig{
+				Command: "test-cmd",
+				Env:     nil,
+			},
+			wantEnv: nil,
+		},
+		{
+			name: "empty Env stays empty",
+			input: &RuntimeConfig{
+				Command: "test-cmd",
+				Env:     map[string]string{},
+			},
+			wantEnv: nil, // Empty map is treated as nil (not copied)
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := fillRuntimeDefaults(tt.input)
+			if result == nil {
+				if !tt.wantNil {
+					t.Fatal("fillRuntimeDefaults returned nil unexpectedly")
+				}
+				return
+			}
+
+			if tt.wantEnv == nil {
+				if result.Env != nil && len(result.Env) > 0 {
+					t.Errorf("expected nil/empty Env, got %v", result.Env)
+				}
+				return
+			}
+
+			if len(result.Env) != len(tt.wantEnv) {
+				t.Errorf("expected %d env vars, got %d", len(tt.wantEnv), len(result.Env))
+			}
+			for k, want := range tt.wantEnv {
+				if got := result.Env[k]; got != want {
+					t.Errorf("Env[%s] = %q, want %q", k, got, want)
+				}
+			}
+		})
+	}
+}
+
+func TestFillRuntimeDefaultsEnvIsCopy(t *testing.T) {
+	t.Parallel()
+	original := &RuntimeConfig{
+		Command: "test-cmd",
+		Env: map[string]string{
+			"ORIGINAL": "value",
+		},
+	}
+
+	result := fillRuntimeDefaults(original)
+
+	// Mutate the result
+	result.Env["MUTATED"] = "yes"
+
+	// Original should be unchanged
+	if _, exists := original.Env["MUTATED"]; exists {
+		t.Error("Mutation of result.Env affected original config")
+	}
+}