feat: adapt to DSH 0.1.2-rc.1 and bump to v0.18.0 (stable)

Graduate from the alpha track to the stable channel:

- Baseline: CI mount pin, dsh.plugin.json engines floor, and every
  @deepseek-ai/* peer floor / devDependencies pin move to 0.1.2-rc.1
  (published on the `next` dist-tag). alpha.5 -> rc.1 is a pure version
  bump in the harness (all package.json "version" lines, zero source
  delta), so no code adaptation; every host contract was re-verified
  unchanged and the real-host mount smoke passes.
- dsh-client-locale resumed publishing; its trailing-pin exception is
  gone — peers and devDeps align at 0.1.2-rc.1 (pnpm peers check clean).
- Version: fold the unpublished v0.18.1-alpha.0 / v0.19.0-alpha.0 bumps
  into the v0.18.0 stable release; SIDEBAR_SERVICE_VERSION follows
  package.json (asserted by tests/service.spec.ts).
- Docs: README/README_EN badges + channel notes + a full "since v0.17.1"
  changelog entry (the two folded alpha numbers are annotated), AGENTS.md
  §2/§3 baseline wording, external-plugin-guide version line, and the
  e2e-host-protocol header's re-verified list gains 0.1.2-rc.1.

tests/e2e-host-protocol.spec.ts

@@ -12,7 +12,7 @@
  *
  * The shapes were transcribed from the deepseek-harness sources at tag
  * dsh-v0.1.2-alpha.1 and re-verified on the npm-published 0.1.2-alpha.2,
- * 0.1.2-alpha.3, and 0.1.2-alpha.5 (packages/client/connection,
+ * 0.1.2-alpha.3, 0.1.2-alpha.5, and 0.1.2-rc.1 (packages/client/connection,
  * packages/api/gateway, packages/bundle/web-app).
  */
 import { beforeAll, describe, expect, it, vi } from 'vitest'