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/web/settings_handlers.go

@@ -6,7 +6,7 @@ import (
 	"path/filepath"
 	"strings"
 
-	"antidrift/internal/settings"
+	"keel/internal/settings"
 
 	"github.com/gin-gonic/gin"
 )
@@ -81,7 +81,7 @@ type browseResponse struct {
 
 // handleBrowse lists subdirectories plus .md files under dir, so the UI can build
 // a file picker that returns a real server-side path. Dotfiles are NOT hidden —
-// the default profile lives under ~/.antidrift. Localhost-only daemon; no jail
+// the default profile lives under ~/.keel. Localhost-only daemon; no jail
 // beyond OS permissions (same trust boundary as the rest of the UI).
 func (s *Server) handleBrowse(c *gin.Context) {
 	dir := strings.TrimSpace(c.Query("dir"))