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/context-types.ts

@@ -87,11 +87,12 @@ export interface SidebarSessionStore {
   get(id: string): {
     header: SidebarSessionHeader
     /**
-     * The live session's append-only event log (immutable snapshot; absent
-     * on sessions the runtime has not hydrated). Read-only access — the
-     * jobs.output route replays `job_output` tool/result rows from it.
+     * The live session's append-only event log as an immutable snapshot.
+     * Read-only access — the jobs.output route replays `job_output`
+     * tool/result rows from it. (The `Session.events` property this face
+     * mirrored was renamed to `snapshotEvents()` in DSH 0.1.2-alpha.4.)
      */
-    events?: readonly SidebarSessionEvent[]
+    snapshotEvents(): readonly SidebarSessionEvent[]
   } | undefined
 }