fix(dsh-plugin-browserskill): serialize per-session commands (observation vs tool calls)

The daemon accepts only one unfinished command per session; the
thumbnail loop racing a model tool call produced 'session already has
an unfinished command' failures on both sides (found by the Phase 1-C
e2e: navigate/snapshot/click all errored while captures kept running).
Add a KeyedExecutor: every command for a session — tool calls and
observation captures alike — runs FIFO; queued tasks reject early on
abort, running tasks keep the runner's signal-driven kill.

packages/dsh-plugin-browserskill/tests/observation.test.ts

@@ -14,6 +14,7 @@ import {
   ObservationService,
 } from "../src/observation";
 import { registerObservationRoutes } from "../src/observation-http";
+import { KeyedExecutor } from "../src/queue";
 import type { BskRunner, BskRunOptions, BskRunResult } from "../src/runner";
 import { SessionRegistry } from "../src/sessions";
 
@@ -128,6 +129,7 @@ function setup(opts: {
     ctx: fakeCtx(attachments),
     runner,
     registry,
+    queue: new KeyedExecutor(),
     options: OPTIONS,
     scheduler,
   });