docs: record the rc.2 baseline (README badges, guide, AGENTS, plan)

- docs/plans/2026-09-10-dsh-0.1.5-rc.2-adaptation.md: the adaptation plan —
  the upstream rc.1 -> rc.2 delta per affected area, why there is no
  plugin-side code change, and the CI failure taxonomy (A real Windows
  timeouts, B the two silent worker deaths, C the mount lane's npm
  ETARGET/OOM) with the evidence and the fix for each.
- AGENTS.md §2/§3: CI pins rc.2, the mount lane's npm flags and their reason
  (floating transitives + staged prerelease publishing), the Windows lane's
  bounded pool and why, and the primitives-dependencies note extended to rc.2.
- README.md / README_EN.md: support badge (v0.19.1, verified on rc.2), the
  install section's stable-release note (rc.1 hosts need no upgrade — the peer
  floor is unchanged) and a v0.19.1 changelog entry covering the two CI fixes.
- docs/external-plugin-guide.md: baseline line -> v0.19.1 / DSH 0.1.5-rc.1+,
  verified on rc.2.
- Comment-only: `DSH 0.1.5-rc.1` -> `0.1.5-rc.1+` where the note describes a
  behaviour that arrived in rc.1 and is unchanged in rc.2 (native guide
  description rendering, in src/client/native/index.ts, src/client/service.ts
  and the three specs that assert it).

src/client/native/index.ts

@@ -65,7 +65,7 @@ function titleOf(descriptor: TabDescriptor): string {
 
 /**
  * The guide fields carrying the descriptor's own description line, evaluated
- * fresh for the current locale. DSH 0.1.5-rc.1 renders `description` only
+ * fresh for the current locale. DSH 0.1.5-rc.1+ renders `description` only
  * while the guide lists at most 4 entries, and a descriptor that declares
  * none must reach the host with NO `description` field at all — the host has
  * no fallback of its own, so an empty string would render as a blank second