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_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")
+	}
+}