backend: add Generate() and Route() stateless one-shot functions

Moves /generate and /route logic from 9p server to core so both
9p and dbus frontends can use them.

pkg/backend/oneshot.go

@@ -0,0 +1,134 @@
+// Package backend — stateless one-shot operations.
+// These functions provide request/response LLM access without session state.
+package backend
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"strings"
+)
+
+// GenerateRequest is the input for a stateless one-shot generation.
+type GenerateRequest struct {
+	Prompt  string `json:"prompt"`
+	System  string `json:"system"`
+	Backend string `json:"backend"`
+	Model   string `json:"model"`
+}
+
+// Generate performs a stateless one-shot LLM generation with no session context.
+// Returns the full model response text.
+func Generate(ctx context.Context, req GenerateRequest) (string, error) {
+	if req.Prompt == "" {
+		return "", fmt.Errorf("empty prompt")
+	}
+	be, err := NewWithName(req.Backend)
+	if err != nil {
+		return "", err
+	}
+	if req.Model != "" {
+		be.SetModel(req.Model)
+	}
+	var msgs []Message
+	if req.System != "" {
+		msgs = append(msgs, Message{Role: "system", Content: req.System})
+	}
+	msgs = append(msgs, Message{Role: "user", Content: req.Prompt})
+	ch, err := be.ChatStream(ctx, msgs, nil, GenerationParams{})
+	if err != nil {
+		return "", err
+	}
+	var sb strings.Builder
+	for ev := range ch {
+		sb.WriteString(ev.Content)
+	}
+	return sb.String(), nil
+}
+
+// RouteRequest is the input for model routing.
+type RouteRequest struct {
+	Task    string `json:"task"`
+	Backend string `json:"backend"`
+}
+
+// RouteResult is the output of model routing.
+type RouteResult struct {
+	Backend string
+	Model   string
+}
+
+// Route classifies a task description and selects the most appropriate model.
+// Uses a cheap classifier model (OLLIE_ROUTE_BACKEND/OLLIE_ROUTE_MODEL) to pick
+// between OLLIE_ROUTE_MODEL_FAST and OLLIE_ROUTE_MODEL_POWER.
+func Route(ctx context.Context, req RouteRequest) (RouteResult, error) {
+	if req.Task == "" {
+		return RouteResult{}, fmt.Errorf("provide a task description")
+	}
+
+	// Classifier backend/model
+	classifyBackend := os.Getenv("OLLIE_ROUTE_BACKEND")
+	if classifyBackend == "" {
+		classifyBackend = "ollama"
+	}
+	classifyModel := os.Getenv("OLLIE_ROUTE_MODEL")
+	if classifyModel == "" {
+		classifyModel = "qwen3:8b"
+	}
+	cbe, err := NewWithName(classifyBackend)
+	if err != nil {
+		return RouteResult{}, fmt.Errorf("route backend: %w", err)
+	}
+	cbe.SetModel(classifyModel)
+
+	// Target backend
+	backendName := req.Backend
+	if backendName == "" {
+		backendName = os.Getenv("OLLIE_BACKEND")
+	}
+	if backendName == "" {
+		backendName = "openrouter"
+	}
+	backendLower := strings.ToLower(backendName)
+
+	// Candidate models
+	fastModel := os.Getenv("OLLIE_ROUTE_MODEL_FAST")
+	powerModel := os.Getenv("OLLIE_ROUTE_MODEL_POWER")
+	if fastModel == "" {
+		fastModel = "mistralai/devstral-small"
+	}
+	if powerModel == "" {
+		powerModel = "anthropic/claude-sonnet-4-20250514"
+	}
+
+	classifyPrompt := fmt.Sprintf(`You are a task router. Given a task description, select the most appropriate model.
+
+Available models:
+1. %s \u2014 fast, cheap, good for: simple edits, single-file changes, straightforward bugs, docs, tests, standard API usage
+2. %s \u2014 powerful, expensive, good for: multi-file architecture, security-sensitive code, complex algorithms, system design, subtle debugging, performance optimization
+
+Reply with ONLY the model name, nothing else.
+
+Task: %s`, fastModel, powerModel, req.Task)
+
+	msgs := []Message{
+		{Role: "user", Content: classifyPrompt},
+	}
+	ch, err := cbe.ChatStream(ctx, msgs, nil, GenerationParams{})
+	if err != nil {
+		return RouteResult{}, fmt.Errorf("route classification failed: %w", err)
+	}
+	var sb strings.Builder
+	for ev := range ch {
+		sb.WriteString(ev.Content)
+	}
+	result := strings.TrimSpace(sb.String())
+
+	// Match result to one of the configured models
+	model := fastModel
+	if strings.Contains(result, powerModel) {
+		model = powerModel
+	}
+
+	return RouteResult{Backend: backendLower, Model: model}, nil
+}