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

src/client/service.ts

@@ -472,7 +472,7 @@ export function matchUrlTarget(tabs: readonly TabDescriptor[], url: URL): TabDes
  * The plugin version this service instance reports. Keep in lockstep with
  * `package.json`'s version — `tests/service.spec.ts` asserts the pair.
  */
-export const SIDEBAR_SERVICE_VERSION = '0.18.1-alpha.0'
+export const SIDEBAR_SERVICE_VERSION = '0.19.0-alpha.0'
 
 /**
  * Monotonic capability list consumers use to gate new API usage (features