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/web/settings_handlers_test.go

@@ -8,7 +8,7 @@ import (
 	"path/filepath"
 	"testing"
 
-	"antidrift/internal/settings"
+	"keel/internal/settings"
 )
 
 // fakeApplier records the last settings it was asked to apply and can be told to
@@ -29,7 +29,7 @@ func (f *fakeApplier) apply(s settings.Settings) error {
 }
 
 func TestGetSettingsReturnsCurrent(t *testing.T) {
-	s := newTestServer(t)
+	s, _ := newTestServer(t)
 	cur := settings.Settings{AIBackend: "claude", MarvinCmd: "am", KnowledgePath: "/tmp/k.md"}
 	fa := &fakeApplier{}
 	s.SetSettings(filepath.Join(t.TempDir(), "settings.json"), cur, fa.apply)
@@ -51,7 +51,7 @@ func TestGetSettingsReturnsCurrent(t *testing.T) {
 }
 
 func TestPostSettingsValidAppliesAndSaves(t *testing.T) {
-	s := newTestServer(t)
+	s, _ := newTestServer(t)
 	path := filepath.Join(t.TempDir(), "settings.json")
 	fa := &fakeApplier{}
 	s.SetSettings(path, settings.Settings{}, fa.apply)
@@ -78,7 +78,7 @@ func TestPostSettingsValidAppliesAndSaves(t *testing.T) {
 }
 
 func TestPostSettingsInvalidBackendIs400AndNoSave(t *testing.T) {
-	s := newTestServer(t)
+	s, _ := newTestServer(t)
 	path := filepath.Join(t.TempDir(), "settings.json")
 	fa := &fakeApplier{reject: true}
 	s.SetSettings(path, settings.Settings{}, fa.apply)
@@ -97,7 +97,7 @@ func TestPostSettingsInvalidBackendIs400AndNoSave(t *testing.T) {
 }
 
 func TestPostSettingsInvalidJSONIs400(t *testing.T) {
-	s := newTestServer(t)
+	s, _ := newTestServer(t)
 	fa := &fakeApplier{}
 	s.SetSettings(filepath.Join(t.TempDir(), "settings.json"), settings.Settings{}, fa.apply)
 	r := s.Router()
@@ -123,7 +123,7 @@ func TestBrowseListsDirsAndMarkdown(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	s := newTestServer(t)
+	s, _ := newTestServer(t)
 	s.SetSettings(filepath.Join(t.TempDir(), "settings.json"), settings.Settings{}, func(settings.Settings) error { return nil })
 	r := s.Router()
 
@@ -179,7 +179,7 @@ func TestBrowseListsDirsAndMarkdown(t *testing.T) {
 }
 
 func TestBrowseBadDirIs400(t *testing.T) {
-	s := newTestServer(t)
+	s, _ := newTestServer(t)
 	s.SetSettings(filepath.Join(t.TempDir(), "settings.json"), settings.Settings{}, func(settings.Settings) error { return nil })
 	r := s.Router()
 	req := httptest.NewRequest(http.MethodGet, "/fs/browse?dir=/no/such/dir/xyz", nil)