feat(dsh-plugin-browserskill): multi-session strip, focus pin, and edge states (Phase 1-C)

- Strip (meeting-style multi-session layout): one item per session
  (mini frame + id + status dot), horizontal row with scroll overflow;
  hover reveals a per-item interrupt button that acts without
  refocusing; click pins the focus view (pin badge, click again to
  release); auto-follow picks the most recently active session but
  never steals focus for errored or dead sessions (red edge / greyed
  item instead).
- Edge states: the host flips an availability flag after repeated
  global capture failures (client shows 'browser unavailable', keeps
  the last frame, greys interrupt, and recovers on the next success);
  a session_not_found envelope marks the session dead (no more frame
  requests, grey strip item, removable as usual); lastError now clears
  when the next action starts or succeeds.
- State/SSE wire gains { available } and an 'availability' event;
  SessionObservation gains dead?.

Tests (+8, 97 total): host availability flip, dead marking +
instrumentation drop + clean removal, lastError lifecycle; client
strip render/pin/unpin/auto-follow, error no-steal, unavailable strip,
dead grey, strip-item interrupt without refocus.

packages/dsh-plugin-browserskill/src/client/ObservationOverlay.module.css

@@ -256,3 +256,101 @@
     animation: none;
   }
 }
+
+.strip {
+  display: flex;
+  gap: 6px;
+  padding: 4px 8px;
+  overflow-x: auto;
+  border-top: 1px solid var(--dsw-alias-border-secondary);
+}
+
+.strip-item {
+  position: relative;
+  flex: none;
+  border-radius: 6px;
+  border: 1px solid var(--dsw-alias-border-secondary);
+}
+
+.strip-item[data-focused] {
+  border-color: var(--dsw-alias-border-focus);
+}
+
+.strip-item[data-state="error"] {
+  border-color: var(--dsw-alias-label-error);
+}
+
+.strip-item[data-state="dead"] {
+  opacity: 0.45;
+}
+
+.strip-main {
+  display: flex;
+  align-items: center;
+  gap: 4px;
+  padding: 3px 6px;
+  border: none;
+  background: none;
+  font: inherit;
+  font-size: 11px;
+  color: var(--dsw-alias-label-secondary);
+  cursor: pointer;
+}
+
+.strip-thumb {
+  width: 28px;
+  height: 20px;
+  overflow: hidden;
+  border-radius: 3px;
+  background: var(--dsw-alias-bg-base);
+  display: inline-flex;
+}
+
+.strip-thumb img {
+  width: 100%;
+  height: 100%;
+  object-fit: cover;
+}
+
+.strip-thumb-empty {
+  width: 100%;
+  height: 100%;
+}
+
+.strip-id {
+  max-width: 64px;
+  overflow: hidden;
+  text-overflow: ellipsis;
+}
+
+.pin-badge {
+  font-size: 9px;
+  line-height: 1;
+}
+
+.strip-interrupt {
+  position: absolute;
+  top: -7px;
+  right: -7px;
+  width: 16px;
+  height: 16px;
+  border-radius: 50%;
+  border: 1px solid var(--dsw-alias-label-error);
+  background: var(--dsw-alias-bg-elevated, var(--dsw-alias-bg-base));
+  color: var(--dsw-alias-label-error);
+  font-size: 8px;
+  line-height: 1;
+  padding: 0;
+  cursor: pointer;
+}
+
+.strip-interrupt:disabled {
+  opacity: 0.4;
+  cursor: default;
+}
+
+.strip-main:focus-visible,
+.strip-interrupt:focus-visible {
+  outline: 2px solid var(--dsw-alias-border-focus);
+  outline-offset: 1px;
+}