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/static/app.css

@@ -88,6 +88,10 @@ input[type="checkbox"] {
 .btn-ghost { background: var(--line); color: var(--ink); }
 .btn:disabled { background: var(--line); color: var(--ink-dim); cursor: not-allowed; }
 
+/* Large tap targets for the launcher and off-screen primary actions. */
+.btn-big { display: block; width: 100%; margin-top: 12px; padding: 14px 16px; font-size: 16px; }
+.launcher .btn-big:first-of-type { margin-top: 4px; }
+
 /* Drift/nudge actions live in the status band; lay them out below the text. */
 .band-actions { margin-top: 10px; display: flex; flex-wrap: wrap; gap: 8px; }
 .band-actions .btn { margin-top: 0; padding: 7px 12px; }
@@ -176,3 +180,13 @@ input[type="checkbox"] {
   font-family: ui-monospace, monospace;
 }
 .browse-list button:hover { background: var(--line); color: var(--accent); }
+
+/* Off-screen mode: pending spinner. */
+.offscreen-pending { text-align: center; }
+.spinner {
+  width: 28px; height: 28px; border-radius: 50%;
+  border: 3px solid var(--line); border-top-color: var(--accent);
+  animation: spin 0.8s linear infinite; margin: 4px auto 10px;
+}
+@keyframes spin { to { transform: rotate(360deg); } }
+@media (prefers-reduced-motion: reduce) { .spinner { animation: none; } }