M2: ai Coach interface and Service over a pluggable Backend

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/ai/coach.go

@@ -0,0 +1,44 @@
+package ai
+
+import "context"
+
+// Coach turns a free-text intent into a validated Proposal.
+type Coach interface {
+	Coach(ctx context.Context, intent string) (Proposal, error)
+}
+
+// Backend is one way to reach an LLM CLI. Adapters differ only in the command
+// and arguments they run; each returns the model's text answer.
+type Backend interface {
+	Run(ctx context.Context, prompt string) (string, error)
+	Name() string
+}
+
+// Service implements Coach over any Backend.
+type Service struct {
+	backend Backend
+}
+
+func NewService(b Backend) *Service { return &Service{backend: b} }
+
+func (s *Service) Coach(ctx context.Context, intent string) (Proposal, error) {
+	out, err := s.backend.Run(ctx, buildPrompt(intent))
+	if err != nil {
+		return Proposal{}, err
+	}
+	return parseProposal(out)
+}
+
+func buildPrompt(intent string) string {
+	return `You are a focus coach. The user gives a rough intent for a work session. Turn it into ONE concrete, actionable commitment.
+
+Respond with ONLY a JSON object, no prose and no code fences, exactly this shape:
+{"next_action": "<one concrete action to start now>", "success_condition": "<observable, verifiable done state>", "timebox_minutes": <integer, typically 15-50>}
+
+Rules:
+- next_action: a single concrete imperative action, doable now.
+- success_condition: observable and verifiable; how you'd know it is done.
+- timebox_minutes: a realistic integer number of minutes for the action.
+
+User intent: ` + intent
+}

internal/ai/coach_test.go

@@ -0,0 +1,49 @@
+package ai
+
+import (
+	"context"
+	"errors"
+	"strings"
+	"testing"
+)
+
+type fakeBackend struct {
+	out       string
+	err       error
+	gotPrompt string
+}
+
+func (f *fakeBackend) Run(ctx context.Context, prompt string) (string, error) {
+	f.gotPrompt = prompt
+	return f.out, f.err
+}
+func (f *fakeBackend) Name() string { return "fake" }
+
+func TestServiceCoachSuccess(t *testing.T) {
+	fb := &fakeBackend{out: `here you go {"next_action":"Write tests","success_condition":"green","timebox_minutes":30}`}
+	svc := NewService(fb)
+	p, err := svc.Coach(context.Background(), "write the tests")
+	if err != nil {
+		t.Fatalf("coach: %v", err)
+	}
+	if p.NextAction != "Write tests" || p.TimeboxSecs != 1800 {
+		t.Fatalf("bad proposal: %+v", p)
+	}
+	if !strings.Contains(fb.gotPrompt, "write the tests") {
+		t.Fatalf("prompt should embed the intent, got: %s", fb.gotPrompt)
+	}
+}
+
+func TestServiceCoachBackendError(t *testing.T) {
+	fb := &fakeBackend{err: errors.New("boom")}
+	if _, err := NewService(fb).Coach(context.Background(), "x"); err == nil {
+		t.Fatal("want backend error")
+	}
+}
+
+func TestServiceCoachUnparseable(t *testing.T) {
+	fb := &fakeBackend{out: "I cannot help with that."}
+	if _, err := NewService(fb).Coach(context.Background(), "x"); !errors.Is(err, ErrNoJSON) {
+		t.Fatalf("want ErrNoJSON, got %v", err)
+	}
+}