feat: adapt to DSH 0.1.2-alpha.5 and bump to v0.19.0-alpha.0

- pins: CI mount-gate CLI, dsh.plugin.json engines floor, and the
  @deepseek-ai/* peer floor / devDependencies baseline move to
  0.1.2-alpha.5 (dsh-client-locale trails at 0.1.2-alpha.3 — no such
  release upstream; ^0.1.2-alpha.3 already accepts the alpha.5 runtime)
- code: migrate the alpha.4 Session.events removal to snapshotEvents()
  (sidechat transcript live reads, fork inheritance, jobs.output replay,
  subagent activity) and drop the seedLength meta field the host removed
  from CreateAgentOptions.meta
- gates: pnpm peers check clean, typecheck + 1137 unit tests green,
  real-host mount smoke 14/14 on npm-published 0.1.2-alpha.5

.github/workflows/ci.yml

@@ -84,7 +84,7 @@ jobs:
       # DSH 0.1.2-alpha.x is published under the `alpha` dist-tag; the lanes
       # speak the slash-only RPC dialect (tests/e2e/host-protocol.ts).
       - name: Install dsh CLI (pinned)
-        run: npm install -g @deepseek-ai/dsh@0.1.2-alpha.3
+        run: npm install -g @deepseek-ai/dsh@0.1.2-alpha.5
 
       # lib/ is gitignored; the tarball is produced from this build.
       - name: Build and pack the npm tarball