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/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)
+}