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/store/store.go

@@ -9,7 +9,7 @@ import (
 	"os"
 	"path/filepath"
 
-	"antidrift/internal/domain"
+	"keel/internal/mode/focus/domain"
 )
 
 // Snapshot is the persisted current state of the daemon.
@@ -29,13 +29,13 @@ type Snapshot struct {
 	EnforcementLevel domain.EnforcementLevel `json:"enforcement_level,omitempty"`
 }
 
-// DefaultPath returns ~/.antidrift/state.json.
+// DefaultPath returns ~/.keel/state.json.
 func DefaultPath() (string, error) {
 	home, err := os.UserHomeDir()
 	if err != nil {
 		return "", err
 	}
-	return filepath.Join(home, ".antidrift", "state.json"), nil
+	return filepath.Join(home, ".keel", "state.json"), nil
 }
 
 // Load reads a snapshot. A missing file yields a default Locked snapshot.