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/agents_test.go

@@ -102,6 +102,21 @@ func TestRuntimeConfigFromPreset(t *testing.T) {
 	}
 }
 
+func TestRuntimeConfigFromPresetReturnsNilEnvForPresetsWithoutEnv(t *testing.T) {
+	t.Parallel()
+	// Built-in presets like Claude don't have Env set
+	// This verifies nil Env handling in RuntimeConfigFromPreset
+	rc := RuntimeConfigFromPreset(AgentClaude)
+	if rc == nil {
+		t.Fatal("RuntimeConfigFromPreset returned nil")
+	}
+
+	// Claude preset doesn't have Env, so it should be nil
+	if rc.Env != nil && len(rc.Env) > 0 {
+		t.Errorf("Expected nil/empty Env for Claude preset, got %v", rc.Env)
+	}
+}
+
 func TestIsKnownPreset(t *testing.T) {
 	t.Parallel()
 	tests := []struct {