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_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)