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).

tests/builtins.spec.ts

@@ -61,7 +61,7 @@ describe('built-in tab registrations', () => {
   })
 
   it('every visible tab declares a non-empty, mutually distinct description', () => {
-    // DSH 0.1.5-rc.1 renders `description` under the title while the guide
+    // DSH 0.1.5-rc.1+ renders `description` under the title while the guide
     // lists at most 4 entries (a longer list drops every description). With
     // the host no longer substituting a generic fallback, a tab without one
     // renders the title alone — so every visible tab declares the real