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/settings/settings.go

@@ -1,5 +1,5 @@
 // Package settings persists the daemon's user-editable configuration as a single
-// JSON file (~/.antidrift/settings.json), parallel to the store snapshot. It is a
+// JSON file (~/.keel/settings.json), parallel to the store snapshot. It is a
 // leaf type package: it imports nothing else in the app so any layer may depend
 // on the Settings value without pulling in adapters.
 package settings
@@ -17,20 +17,20 @@ import (
 var ErrInvalidBackend = errors.New("settings: invalid ai backend")
 
 // Settings is the user-editable configuration. Field names mirror the env vars
-// they replace: ANTIDRIFT_AI_BACKEND, ANTIDRIFT_MARVIN_CMD, ANTIDRIFT_KNOWLEDGE_FILE.
+// they replace: KEEL_AI_BACKEND, KEEL_MARVIN_CMD, KEEL_KNOWLEDGE_FILE.
 type Settings struct {
 	AIBackend     string `json:"ai_backend"`
 	MarvinCmd     string `json:"marvin_cmd"`
 	KnowledgePath string `json:"knowledge_path"`
 }
 
-// DefaultPath returns ~/.antidrift/settings.json.
+// DefaultPath returns ~/.keel/settings.json.
 func DefaultPath() (string, error) {
 	home, err := os.UserHomeDir()
 	if err != nil {
 		return "", err
 	}
-	return filepath.Join(home, ".antidrift", "settings.json"), nil
+	return filepath.Join(home, ".keel", "settings.json"), nil
 }
 
 // Load reads a settings file. A missing file is an error; callers treat that as
@@ -64,17 +64,17 @@ func Save(path string, s Settings) error {
 	return os.Rename(tmp, path)
 }
 
-// SeedFromEnv builds a Settings from the legacy ANTIDRIFT_* env vars, applying
+// SeedFromEnv builds a Settings from the KEEL_* env vars, applying
 // the same defaults the daemon used before the settings file existed. An unset
 // AI backend defaults to "claude" (matching ai.NewBackend("")).
 func SeedFromEnv() Settings {
-	backend := os.Getenv("ANTIDRIFT_AI_BACKEND")
+	backend := os.Getenv("KEEL_AI_BACKEND")
 	if backend == "" {
 		backend = "claude"
 	}
 	return Settings{
 		AIBackend:     backend,
-		MarvinCmd:     os.Getenv("ANTIDRIFT_MARVIN_CMD"),
-		KnowledgePath: os.Getenv("ANTIDRIFT_KNOWLEDGE_FILE"),
+		MarvinCmd:     os.Getenv("KEEL_MARVIN_CMD"),
+		KnowledgePath: os.Getenv("KEEL_KNOWLEDGE_FILE"),
 	}
 }

internal/settings/settings_test.go

@@ -29,9 +29,9 @@ func TestLoadMissingFileIsError(t *testing.T) {
 }
 
 func TestSeedFromEnvReadsVars(t *testing.T) {
-	t.Setenv("ANTIDRIFT_AI_BACKEND", "codex")
-	t.Setenv("ANTIDRIFT_MARVIN_CMD", "uv run am")
-	t.Setenv("ANTIDRIFT_KNOWLEDGE_FILE", "/tmp/k.md")
+	t.Setenv("KEEL_AI_BACKEND", "codex")
+	t.Setenv("KEEL_MARVIN_CMD", "uv run am")
+	t.Setenv("KEEL_KNOWLEDGE_FILE", "/tmp/k.md")
 	got := SeedFromEnv()
 	want := Settings{AIBackend: "codex", MarvinCmd: "uv run am", KnowledgePath: "/tmp/k.md"}
 	if got != want {
@@ -40,9 +40,9 @@ func TestSeedFromEnvReadsVars(t *testing.T) {
 }
 
 func TestSeedFromEnvDefaultsBackend(t *testing.T) {
-	t.Setenv("ANTIDRIFT_AI_BACKEND", "")
-	t.Setenv("ANTIDRIFT_MARVIN_CMD", "")
-	t.Setenv("ANTIDRIFT_KNOWLEDGE_FILE", "")
+	t.Setenv("KEEL_AI_BACKEND", "")
+	t.Setenv("KEEL_MARVIN_CMD", "")
+	t.Setenv("KEEL_KNOWLEDGE_FILE", "")
 	got := SeedFromEnv()
 	if got.AIBackend != "claude" {
 		t.Errorf("default backend = %q, want claude", got.AIBackend)