Add tasks port and Amazing Marvin adapter

The tasks.Provider port answers "what should I be doing?". The Marvin
adapter shells out to ampy's `am --json` and parses today's open tasks,
dropping done/empty-title entries. Leaf package mirroring ai.

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

internal/tasks/marvin.go

@@ -0,0 +1,85 @@
+package tasks
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"os/exec"
+	"strings"
+)
+
+// runner executes a command and returns its stdout. Production shells out;
+// tests inject a fake to avoid spawning a process.
+type runner func(ctx context.Context, name string, args ...string) ([]byte, error)
+
+func execRunner(ctx context.Context, name string, args ...string) ([]byte, error) {
+	cmd := exec.CommandContext(ctx, name, args...)
+	var out, errb bytes.Buffer
+	cmd.Stdout = &out
+	cmd.Stderr = &errb
+	if err := cmd.Run(); err != nil {
+		if s := strings.TrimSpace(errb.String()); s != "" {
+			return nil, fmt.Errorf("%s: %w: %s", name, err, s)
+		}
+		return nil, fmt.Errorf("%s: %w", name, err)
+	}
+	return out.Bytes(), nil
+}
+
+// Marvin is the Amazing Marvin adapter. It shells out to ampy's CLI
+// (`am --json`, no subcommand) and parses the open tasks due today or earlier.
+type Marvin struct {
+	cmd  string
+	args []string
+	run  runner
+}
+
+// NewMarvin builds the adapter. command is split on spaces so both "am" and
+// "uv run am" work; empty defaults to "am". The "--json" flag is always
+// appended, and no subcommand is passed (ampy lists today's tasks by default).
+func NewMarvin(command string) *Marvin {
+	fields := strings.Fields(command)
+	if len(fields) == 0 {
+		fields = []string{"am"}
+	}
+	args := append([]string{}, fields[1:]...)
+	args = append(args, "--json")
+	return &Marvin{cmd: fields[0], args: args, run: execRunner}
+}
+
+// Today returns the open tasks due today or earlier, or an error if the CLI
+// fails. Callers degrade gracefully on error.
+func (m *Marvin) Today(ctx context.Context) ([]Task, error) {
+	out, err := m.run(ctx, m.cmd, m.args...)
+	if err != nil {
+		return nil, err
+	}
+	return parse(out)
+}
+
+type rawTask struct {
+	ID    string `json:"id"`
+	Title string `json:"title"`
+	Day   string `json:"day"`
+	Done  bool   `json:"done"`
+}
+
+// parse maps `am --json` output (a JSON array of task objects) to []Task. It
+// drops tasks that are done or have an empty title. A JSON null or empty array
+// yields an empty slice, not an error.
+func parse(data []byte) ([]Task, error) {
+	var raw []rawTask
+	if err := json.Unmarshal(data, &raw); err != nil {
+		return nil, fmt.Errorf("tasks: parse: %w", err)
+	}
+	out := make([]Task, 0, len(raw))
+	for _, r := range raw {
+		title := strings.TrimSpace(r.Title)
+		if r.Done || title == "" {
+			continue
+		}
+		out = append(out, Task{ID: r.ID, Title: title, Day: r.Day})
+	}
+	return out, nil
+}

internal/tasks/marvin_test.go

@@ -0,0 +1,108 @@
+package tasks
+
+import (
+	"context"
+	"errors"
+	"testing"
+)
+
+func TestParse(t *testing.T) {
+	cases := []struct {
+		name    string
+		in      string
+		want    []Task
+		wantErr bool
+	}{
+		{
+			name: "valid array",
+			in:   `[{"id":"a","title":"Write spec","parentId":"p","day":"2026-05-31","done":false}]`,
+			want: []Task{{ID: "a", Title: "Write spec", Day: "2026-05-31"}},
+		},
+		{name: "empty array", in: `[]`, want: []Task{}},
+		{name: "json null", in: `null`, want: []Task{}},
+		{
+			name: "drops done and empty-title",
+			in:   `[{"id":"a","title":"keep","day":"","done":false},{"id":"b","title":"done one","done":true},{"id":"c","title":"  ","done":false}]`,
+			want: []Task{{ID: "a", Title: "keep", Day: ""}},
+		},
+		{name: "malformed", in: `not json`, wantErr: true},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			got, err := parse([]byte(tc.in))
+			if tc.wantErr {
+				if err == nil {
+					t.Fatal("want error, got nil")
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("parse: %v", err)
+			}
+			if len(got) != len(tc.want) {
+				t.Fatalf("len = %d, want %d (%+v)", len(got), len(tc.want), got)
+			}
+			for i := range got {
+				if got[i] != tc.want[i] {
+					t.Errorf("[%d] = %+v, want %+v", i, got[i], tc.want[i])
+				}
+			}
+		})
+	}
+}
+
+func TestNewMarvinCommandSplitting(t *testing.T) {
+	cases := []struct {
+		in       string
+		wantCmd  string
+		wantArgs []string
+	}{
+		{"", "am", []string{"--json"}},
+		{"am", "am", []string{"--json"}},
+		{"uv run am", "uv", []string{"run", "am", "--json"}},
+	}
+	for _, tc := range cases {
+		m := NewMarvin(tc.in)
+		if m.cmd != tc.wantCmd {
+			t.Errorf("NewMarvin(%q).cmd = %q, want %q", tc.in, m.cmd, tc.wantCmd)
+		}
+		if len(m.args) != len(tc.wantArgs) {
+			t.Fatalf("NewMarvin(%q).args = %v, want %v", tc.in, m.args, tc.wantArgs)
+		}
+		for i := range m.args {
+			if m.args[i] != tc.wantArgs[i] {
+				t.Errorf("NewMarvin(%q).args[%d] = %q, want %q", tc.in, i, m.args[i], tc.wantArgs[i])
+			}
+		}
+	}
+}
+
+func TestTodayUsesRunnerOutput(t *testing.T) {
+	m := NewMarvin("am")
+	var gotName string
+	var gotArgs []string
+	m.run = func(ctx context.Context, name string, args ...string) ([]byte, error) {
+		gotName, gotArgs = name, args
+		return []byte(`[{"id":"x","title":"Do thing","day":"2026-05-31","done":false}]`), nil
+	}
+	got, err := m.Today(context.Background())
+	if err != nil {
+		t.Fatalf("Today: %v", err)
+	}
+	if gotName != "am" || len(gotArgs) != 1 || gotArgs[0] != "--json" {
+		t.Fatalf("runner called with %q %v", gotName, gotArgs)
+	}
+	if len(got) != 1 || got[0].Title != "Do thing" {
+		t.Fatalf("Today result = %+v", got)
+	}
+}
+
+func TestTodayPropagatesError(t *testing.T) {
+	m := NewMarvin("am")
+	m.run = func(ctx context.Context, name string, args ...string) ([]byte, error) {
+		return nil, errors.New("am not found")
+	}
+	if _, err := m.Today(context.Background()); err == nil {
+		t.Fatal("want error from failing runner")
+	}
+}

internal/tasks/tasks.go

@@ -0,0 +1,19 @@
+// Package tasks is the Tasks port: it answers "what should I be doing?" by
+// listing the open to-do items due today or earlier. It imports nothing from
+// the rest of the app, so it stays a leaf package.
+package tasks
+
+import "context"
+
+// Task is one to-do item. Primitives only, so tasks stays a leaf package.
+type Task struct {
+	ID    string
+	Title string
+	Day   string // "YYYY-MM-DD", or "" if unscheduled
+}
+
+// Provider answers "what should I be doing?" — the open tasks due today or
+// earlier.
+type Provider interface {
+	Today(ctx context.Context) ([]Task, error)
+}