Generalize the focus controller into a harness hosting swappable modes

Loosen AntiDrift's session controller into Keel's general collect→brain→act
loop. A new internal/harness runs at most one mode.Mode at a time, fanning
async completions out to the web SSE and status-bar surfaces.

- internal/mode: the Mode contract (Kind/Command/View/Active) plus optional
  EvidenceConsumer and Expirer ports, and the surfacing Envelope.
- internal/mode/focus: the former session/domain/statemachine packages moved
  under the mode, now satisfying the harness contracts unchanged.
- internal/mode/offscreen: a one-shot away-from-desk mode built on the new
  ai.Proposer, which turns a life-domain brief into one off-screen action.
- cmd/keeld replaces cmd/antidriftd; daemon wires focus + offscreen factories
  with per-mode persistence under ~/.keel/modes/<kind>.
- Finish the rename: KEEL_* env refs in the README, /keeld build artifact
  ignored, stale antidriftd binary removed.

Include the design + implementation plan this refactor was built from under
docs/superpowers/{specs,plans}/2026-06-04-controller-refactor*.md.

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

internal/tasks/marvin_test.go

@@ -3,6 +3,7 @@ package tasks
 import (
 	"context"
 	"errors"
+	"strings"
 	"testing"
 )
 
@@ -106,3 +107,35 @@ func TestTodayPropagatesError(t *testing.T) {
 		t.Fatal("want error from failing runner")
 	}
 }
+
+func TestCreateInvokesMarvinAddPreservingCRDT(t *testing.T) {
+	m := NewMarvin("uv run 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":"task-1"}`), nil
+	}
+	if err := m.Create(context.Background(), NewTask("buy groceries")); err != nil {
+		t.Fatalf("Create: %v", err)
+	}
+	// Same prefix the read path shells out through, so add goes through the
+	// same ampy entrypoint and preserves its CRDT bookkeeping.
+	if gotName != "uv" {
+		t.Fatalf("runner name = %q, want %q", gotName, "uv")
+	}
+	joined := strings.Join(gotArgs, " ")
+	if !strings.Contains(joined, "add") || !strings.Contains(joined, "buy groceries") {
+		t.Fatalf("args = %v, want marvin add with title", gotArgs)
+	}
+}
+
+func TestCreatePropagatesError(t *testing.T) {
+	m := NewMarvin("am")
+	m.run = func(ctx context.Context, name string, args ...string) ([]byte, error) {
+		return nil, errors.New("am add failed")
+	}
+	if err := m.Create(context.Background(), NewTask("buy groceries")); err == nil {
+		t.Fatal("want error from failing runner")
+	}
+}