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/harness/services.go

@@ -0,0 +1,24 @@
+package harness
+
+import (
+	"time"
+
+	"keel/internal/ai"
+	"keel/internal/enforce"
+	"keel/internal/knowledge"
+	"keel/internal/tasks"
+)
+
+// Services is the shared infrastructure every mode receives from the harness.
+// Dir is the mode's namespaced persistence directory (~/.keel/modes/<kind>);
+// modes build their own file paths under it. Notify fires the harness change
+// listeners (web SSE + status bar).
+type Services struct {
+	AI        *ai.Service
+	Tasks     tasks.Provider
+	Knowledge knowledge.Source
+	Enforce   enforce.Guard
+	Clock     func() time.Time
+	Dir       string
+	Notify    func()
+}