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/knowledge/file.go

@@ -22,7 +22,7 @@ type FileSource struct {
 }
 
 // NewFileSource builds the adapter. defaultPath is used when Load receives an
-// empty path; if it too is empty, Load falls back to ~/.antidrift/knowledge.md.
+// empty path; if it too is empty, Load falls back to ~/.keel/knowledge.md.
 func NewFileSource(defaultPath string) *FileSource {
 	return &FileSource{defaultPath: defaultPath}
 }
@@ -46,7 +46,7 @@ func (s *FileSource) Load(ctx context.Context, path string) (Profile, error) {
 	return Profile{Text: truncate(text, maxProfileBytes), Path: resolved}, nil
 }
 
-// resolve picks path, else the default, else ~/.antidrift/knowledge.md; expands
+// resolve picks path, else the default, else ~/.keel/knowledge.md; expands
 // a leading ~; and makes the result absolute for stable display.
 func (s *FileSource) resolve(path string) string {
 	p := path
@@ -55,7 +55,7 @@ func (s *FileSource) resolve(path string) string {
 	}
 	if p == "" {
 		if home, err := os.UserHomeDir(); err == nil {
-			p = filepath.Join(home, ".antidrift", "knowledge.md")
+			p = filepath.Join(home, ".keel", "knowledge.md")
 		}
 	}
 	p = expandTilde(p)