agent: layered preamble scaffolding

BuildRuntime now accepts optional baseLayers (variadic strings) that
are prepended to the agent-resolved preamble. This enables the server
to compose: system_prompt + operational_model + environment + agent_prompts.

- config.Config: add SystemPrompt field (path override)
- AgentCoreConfig: add OperationalModel and SystemPrompt fields
- BuildRuntime: prepend baseLayers before agent prompt content
- Remove debug fprintf statements from prompt resolution

pkg/agent/core.go

@@ -38,7 +38,7 @@ type toolClassifier func(name string) bool
 // env provides additional environment variables injected into prompt resolution
 // subprocesses (e.g. OLLIE_SESSION_ID=xxx).
 // The caller is responsible for registering all servers on d before calling this.
-func BuildRuntime(cfg *config.Config, d tools.Dispatcher, cwd string, env []string) *Runtime {
+func BuildRuntime(cfg *config.Config, d tools.Dispatcher, cwd string, env []string, baseLayers ...string) *Runtime {
 	var messages []string
 
 	var allToolInfos []tools.ToolInfo
@@ -65,11 +65,9 @@ func BuildRuntime(cfg *config.Config, d tools.Dispatcher, cwd string, env []stri
 		for k, v := range cfg.Hooks {
 			hooks[k] = []string(v)
 		}
-		fmt.Fprintf(os.Stderr, "[DEBUG BuildRuntime] cfg.Prompt.IsExec=%v len(Value)=%d\n", cfg.Prompt.IsExec, len(cfg.Prompt.Value))
 		if resolved, err := resolvePrompt(cfg.Prompt, cwd, env); err != nil {
 			fmt.Fprintf(os.Stderr, "resolve prompt: %v\n", err)
 		} else {
-			fmt.Fprintf(os.Stderr, "[DEBUG BuildRuntime] preamble len=%d\n", len(resolved))
 			preamble = resolved
 		}
 		genParams = backend.GenerationParams{
@@ -147,6 +145,21 @@ func BuildRuntime(cfg *config.Config, d tools.Dispatcher, cwd string, env []stri
 		compactionModel = cfg.CompactionModel
 	}
 
+	// Prepend base layers (system prompt, operational model, environment)
+	// before the agent-specific preamble.
+	if len(baseLayers) > 0 {
+		var prefix strings.Builder
+		for _, layer := range baseLayers {
+			if layer != "" {
+				prefix.WriteString(layer)
+				prefix.WriteByte('\n')
+			}
+		}
+		if prefix.Len() > 0 {
+			preamble = prefix.String() + preamble
+		}
+	}
+
 	return &Runtime{
 		Dispatcher:      d,
 		Tools:           allTools,
@@ -251,6 +264,13 @@ type AgentCoreConfig struct {
 	PromptEnvExtra []string
 	// Remote is the SSH target for remote execution (empty = local).
 	Remote string
+	// OperationalModel is server-injected text describing the 9P/D-Bus
+	// interface available to the agent. Not user-configurable.
+	OperationalModel string
+	// SystemPrompt is the base system prompt text (embedded default or
+	// override from agent config / session creation). If empty, BuildRuntime
+	// uses only the agent config's prompt commands.
+	SystemPrompt string
 }
 
 // agent is the Core implementation. It owns all agent and session state

pkg/agent/prompt_resolver.go

@@ -71,16 +71,13 @@ func resolveExecPrompt(cmds []string, cwd string, env []string) (string, error)
 	}
 
 	var parts []string
-	fmt.Fprintf(os.Stderr, "[DEBUG resolveExecPrompt] %d entries\n", len(cmds))
 	for _, entry := range cmds {
 		entry = strings.TrimSpace(entry)
 		if entry == "" {
 			continue
 		}
-		fmt.Fprintf(os.Stderr, "[DEBUG resolveExecPrompt] entry=%q\n", entry)
 		// Try resolving as a prompt file first.
 		if content, ok := resolvePromptFile(entry, envMap, expand); ok {
-			fmt.Fprintf(os.Stderr, "[DEBUG resolveExecPrompt] -> file ok, len=%d\n", len(content))
 			if content != "" {
 				parts = append(parts, content)
 			}

pkg/config/config.go

@@ -85,6 +85,9 @@ type Config struct {
 	Stop              []string `json:"stop,omitempty"`
 	Verbosity         string   `json:"verbosity,omitempty"`
 	CompactionModel   string   `json:"compactionModel,omitempty"`
+	// SystemPrompt overrides the embedded system prompt with a file path.
+	// If set and the file exists, its content replaces the compiled-in default.
+	SystemPrompt      string   `json:"systemPrompt,omitempty"`
 }
 
 // Load parses a Config from r.