feat(runner): append spec instructions to llm system prompt

internal/runner/llm_source.go

@@ -52,8 +52,11 @@ type llmSource struct {
 	verifier *verifier.Verifier
 	client   *llmclient.Client
 	model    string
-	logger   *slog.Logger
-	history  *actionHistory
+	// instructions is optional spec-level guidance appended to the system prompt
+	// to steer the model's bug-hunting (empty when unset).
+	instructions string
+	logger       *slog.Logger
+	history      *actionHistory
 
 	// lastSource/lastReasoning describe the most recent NextAction so the runner
 	// can stamp the trace. lastSource is "llm" only when the LLM (not setup)
@@ -159,13 +162,23 @@ func (s *llmSource) buildRequest(candidates []verifier.ActionCandidate) llmclien
 	return llmclient.Request{
 		Model: s.model,
 		Messages: []llmclient.Message{
-			{Role: "system", Content: []llmclient.ContentPart{llmclient.TextPart(llmSystemPrompt)}},
+			{Role: "system", Content: []llmclient.ContentPart{llmclient.TextPart(s.systemPrompt())}},
 			{Role: "user", Content: userParts},
 		},
 		ResponseFormat: rankedResponseFormat(),
 	}
 }
 
+// systemPrompt is the base framing plus any spec-level instructions, appended as
+// extra guidance so a spec can steer the model's bug-hunting without losing the
+// candidate-kind semantics the base prompt establishes.
+func (s *llmSource) systemPrompt() string {
+	if strings.TrimSpace(s.instructions) == "" {
+		return llmSystemPrompt
+	}
+	return llmSystemPrompt + "\n\n" + s.instructions
+}
+
 // userPrompt renders the numbered candidate list and the recent-action memory.
 func (s *llmSource) userPrompt(candidates []verifier.ActionCandidate) string {
 	var builder strings.Builder

internal/runner/llm_source_test.go

@@ -141,6 +141,19 @@ func TestParseRanked(t *testing.T) {
 	}
 }
 
+func TestSystemPromptAppendsInstructions(t *testing.T) {
+	if got := (&llmSource{}).systemPrompt(); got != llmSystemPrompt {
+		t.Error("empty instructions should yield the base prompt unchanged")
+	}
+	withInstr := (&llmSource{instructions: "hunt for double submits"}).systemPrompt()
+	if !strings.Contains(withInstr, llmSystemPrompt) {
+		t.Error("system prompt must retain the base framing")
+	}
+	if !strings.Contains(withInstr, "hunt for double submits") {
+		t.Error("system prompt must include the spec instructions")
+	}
+}
+
 // fakeOpenRouter is a configurable in-process OpenRouter server. Set ranked /
 // reasoning before each call; it echoes them as a json_schema content body.
 type fakeOpenRouter struct {

internal/runner/source.go

@@ -83,11 +83,12 @@ func pickSources(options Options) (ActionSource, ExtractorSource, error) {
 			logger = slog.Default()
 		}
 		action := &llmSource{
-			verifier: options.Verifier,
-			client:   client,
-			model:    config.Model,
-			logger:   logger,
-			history:  newActionHistory(llmHistorySize),
+			verifier:     options.Verifier,
+			client:       client,
+			model:        config.Model,
+			instructions: config.Instructions,
+			logger:       logger,
+			history:      newActionHistory(llmHistorySize),
 		}
 		return action, gojaSource{verifier: options.Verifier}, nil
 	}