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/mode.go

@@ -0,0 +1,44 @@
+// Package mode defines the contract the harness uses to host a single
+// collect→brain→act unit. Focus and off-screen are implementations.
+package mode
+
+import (
+	"context"
+	"encoding/json"
+	"time"
+
+	"keel/internal/evidence"
+)
+
+// Mode is one collect→brain→act configuration. The harness runs at most one.
+type Mode interface {
+	// Kind identifies the mode ("focus", "offscreen").
+	Kind() string
+	// Command routes a surface-issued command into the mode. It returns an
+	// error for illegal/invalid commands; the web layer maps it to HTTP.
+	Command(ctx context.Context, name string, payload json.RawMessage) error
+	// View projects the mode's current state for surfaces (JSON-marshalable).
+	View() any
+	// Active reports whether the mode still holds the harness. When it returns
+	// false the harness releases the mode and goes idle.
+	Active() bool
+}
+
+// EvidenceConsumer is implemented by modes that read the window sensor stream.
+type EvidenceConsumer interface {
+	OnWindow(evidence.WindowSnapshot)
+}
+
+// Expirer is implemented by modes with a server-authoritative deadline. Expire
+// takes no context to match focus's existing Expire() error method, so focus
+// satisfies this interface with zero changes to its tested surface.
+type Expirer interface {
+	Deadline() time.Time
+	Expire() error
+}
+
+// Envelope is the surfacing wrapper: the active mode's kind plus its View().
+type Envelope struct {
+	ActiveMode string `json:"active_mode"` // "" = idle
+	Mode       any    `json:"mode,omitempty"`
+}