fix(dsh-plugin-browserskill): address review findings (leaks, trust fence, queue race)

Blocking:
- B1: delete screenshot scratch PNGs — observation frames reuse one fixed
  per-session path and unlink after every read (finally); browser_screenshot
  unlinks once the bytes are in the attachment store (kept only when the file
  itself is the model-facing artifact).
- B2: replicate dsh's browser-trust fence on /bsk-observation/*: loopback
  Host only, Origin must match Host, sec-fetch-site: cross-site refused, POST
  requires application/json; README documents the loopback trust premise and
  the 0.0.0.0 warning.

Major:
- M1: KeyedExecutor tail chains previous+task (allSettled) so a task aborted
  while queued cannot release the next one into the running session.
- M2: client tracks the live frame per session — replacing/removing/reset
  revokes the old blob URL at once; loads settling after replacement never
  resurrect it.

Minor: drop replaced thumbRefs (+on remove); beginAction fires inside the
queue (no label overwrite while queued, no idle flash); half-initialized
session cleanup stops through the queue with one retry; SSE (re)open refetches
/state (+ snapshot flag renamed subscribed); install probe uses --version (no
daemon spawn); emulate mobile documents the width+height requirement (the
daemon refuses it alone — verified).

Nit: single BskRunOptions declaration; tails map entry dropped on drain; SSE
cleanup on res close; popOut catches requestWindow rejections; resize handle
gains keyboard control (arrows, 16px steps, aria value attrs).

Tests: +14 (fence rules, scratch lifecycle, queue race, blob revocation,
instrumentation timing, SSE resync). 113/113 green.

packages/dsh-plugin-browserskill/README.md

@@ -107,6 +107,14 @@ shell's theme cannot bleed back in.
   the dsh `webServer` route seam (dsh 0.1's Typert Remote pipeline is closed to out-of-tree
   packages). All commands for one session — tool calls and frame captures alike — run through a
   per-session FIFO, because the daemon accepts only one unfinished command per session.
+- **Trust model**: these routes expose live screenshots (and an interrupt write), so they
+  replicate the browser-trust fence dsh applies to its own `/api` routes: the request Host
+  must be a loopback authority (`localhost`, `127.0.0.0/8`, `[::1]`), a present Origin must
+  match the Host, `sec-fetch-site: cross-site` is refused, and POST requires an
+  `application/json` body (cross-site simple requests can never satisfy that). The channel is
+  therefore built for **loopback-only serving** — binding the dsh web server to `0.0.0.0` and
+  reaching it through a LAN address will (deliberately) fail the fence; do not put these
+  routes behind a non-loopback reverse proxy without adding your own authentication.
 - Configure with `observationEnabled` / `thumbnailIntervalMs` / `idleIntervalMs`.
 
 ## Behavior notes