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/mode/focus/factory.go

@@ -0,0 +1,50 @@
+package focus
+
+import (
+	"path/filepath"
+
+	"keel/internal/harness"
+	"keel/internal/mode"
+)
+
+// build constructs a focus Mode under the mode's namespaced dir and injects the
+// shared services through the existing setters (the same API tests use).
+func build(svc harness.Services) (*Mode, error) {
+	m, err := New(filepath.Join(svc.Dir, "state.json"))
+	if err != nil {
+		return nil, err
+	}
+	if svc.Clock != nil {
+		m.SetClock(svc.Clock)
+	}
+	m.SetCoach(svc.AI)
+	m.SetReviewer(svc.AI)
+	m.SetDriftJudge(svc.AI)
+	m.SetNudge(svc.AI)
+	m.SetTasks(svc.Tasks)
+	m.SetKnowledge(svc.Knowledge)
+	m.SetGuard(svc.Enforce)
+	m.SetOnChange(svc.Notify) // harness fan-out becomes focus's change listener
+	return m, nil
+}
+
+// Factory builds a fresh focus mode entering Planning (used by harness.Start).
+func Factory(svc harness.Services) mode.Mode {
+	m, err := build(svc)
+	if err != nil {
+		m, _ = New("") // degrade to an in-memory Locked mode; EnterPlanning still works
+		m.SetOnChange(svc.Notify)
+	}
+	_ = m.EnterPlanning()
+	return m
+}
+
+// Restore loads a persisted focus session and reports whether one was live
+// (Planning/Active/Transition/Review). Used at startup to re-adopt a session.
+func Restore(svc harness.Services) (*Mode, bool, error) {
+	m, err := build(svc)
+	if err != nil {
+		return nil, false, err
+	}
+	return m, m.Active(), nil
+}