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
+}