Decompose App God struct into embedded section structs

Replace 60+ flat fields in App with 4 embedded section structs:
CoreConfig, ProviderConfig, PromptConfig, FeatureConfig. Go field
promotion preserves all existing field access patterns. Updated
flattenAppConfig to recurse into embedded structs for runtimeconfig.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

2 files changed, 25 insertions, 13 deletions

diff --git a/internal/hexaiaction/parse_test.go b/internal/hexaiaction/parse_test.go
index ba5cd96..40ddd9a 100644
--- a/internal/hexaiaction/parse_test.go
+++ b/internal/hexaiaction/parse_test.go
@@ -81,14 +81,16 @@ func (f *fakeClient) DefaultModel() string { return "m" }
 
 func TestRuners_Prompts(t *testing.T) {
 	cfg := appconfig.App{
-		PromptCodeActionRewriteSystem:     "SYS-R",
-		PromptCodeActionRewriteUser:       "R {{instruction}} :: {{selection}}",
-		PromptCodeActionDiagnosticsSystem: "SYS-D",
-		PromptCodeActionDiagnosticsUser:   "D {{diagnostics}} :: {{selection}}",
-		PromptCodeActionDocumentSystem:    "SYS-C",
-		PromptCodeActionDocumentUser:      "C {{selection}}",
-		PromptCodeActionGoTestSystem:      "SYS-T",
-		PromptCodeActionGoTestUser:        "T {{function}}",
+		PromptConfig: appconfig.PromptConfig{
+			PromptCodeActionRewriteSystem:     "SYS-R",
+			PromptCodeActionRewriteUser:       "R {{instruction}} :: {{selection}}",
+			PromptCodeActionDiagnosticsSystem: "SYS-D",
+			PromptCodeActionDiagnosticsUser:   "D {{diagnostics}} :: {{selection}}",
+			PromptCodeActionDocumentSystem:    "SYS-C",
+			PromptCodeActionDocumentUser:      "C {{selection}}",
+			PromptCodeActionGoTestSystem:      "SYS-T",
+			PromptCodeActionGoTestUser:        "T {{function}}",
+		},
 	}
 	f := &fakeClient{out: "```\nDONE\n```"}
 	ctx := context.Background()
diff --git a/internal/hexaiaction/prompts_more_test.go b/internal/hexaiaction/prompts_more_test.go
index a4410e5..844bafe 100644
--- a/internal/hexaiaction/prompts_more_test.go
+++ b/internal/hexaiaction/prompts_more_test.go
@@ -33,10 +33,14 @@ func TestRunOnce_StripsFences(t *testing.T) {
 
 func TestReqOptsFrom_Override(t *testing.T) {
 	cfg := appconfig.App{
-		MaxTokens:         123,
-		Provider:          "openai",
-		AnthropicModel:    "claude-3-5-sonnet",
-		CodeActionConfigs: []appconfig.SurfaceConfig{{Provider: "anthropic", Model: "override", Temperature: ptrFloat(0.6)}},
+		CoreConfig: appconfig.CoreConfig{
+			MaxTokens: 123,
+			Provider:  "openai",
+		},
+		ProviderConfig: appconfig.ProviderConfig{
+			AnthropicModel:    "claude-3-5-sonnet",
+			CodeActionConfigs: []appconfig.SurfaceConfig{{Provider: "anthropic", Model: "override", Temperature: ptrFloat(0.6)}},
+		},
 	}
 	req := reqOptsFrom(cfg)
 	if req.model != "override" {
@@ -52,7 +56,13 @@ func TestReqOptsFrom_Override(t *testing.T) {
 }
 
 func TestReqOptsFrom_Gpt5Temp(t *testing.T) {
-	cfg := appconfig.App{Provider: "openai", CodingTemperature: ptrFloat(0.2), OpenAIModel: "gpt-5.0"}
+	cfg := appconfig.App{
+		CoreConfig: appconfig.CoreConfig{
+			Provider:          "openai",
+			CodingTemperature: ptrFloat(0.2),
+		},
+		ProviderConfig: appconfig.ProviderConfig{OpenAIModel: "gpt-5.0"},
+	}
 	req := reqOptsFrom(cfg)
 	var opts llm.Options
 	for _, o := range req.options {