feat(session-query): ship full-text session search opt-in via openAt never

The shipped bundles keep ctx.sessionQuery mounted but set the new
session-query-sqlite `openAt: never` phase: searchSessions/searchEvents
fail with the typed SESSION_QUERY_SEARCH_DISABLED code before any request
normalization, node:sqlite is never imported or opened, and no source
observation or reconciliation runs. Every inherited exact read, filter,
and trace — session export descendants, subagent-fork Workspace
inheritance, title reads — keeps working, and the Web sidebar search
degrades to its designed local title/workspace matching. Enabling content
search is a one-line openAt override in a later patch layer; the web e2e
scaffold keeps it enabled as the assembled opt-in coverage.

packages/session-query/tool-session-query/src/service-boundary.ts

@@ -71,6 +71,10 @@ const SAFE_SESSION_QUERY_FAILURES = {
     code: 'SESSION_QUERY_PERSISTENCE_FAILED',
     message: 'session history storage is unavailable',
   },
+  SESSION_QUERY_SEARCH_DISABLED: {
+    code: 'SESSION_QUERY_SEARCH_DISABLED',
+    message: 'session search is disabled in this deployment',
+  },
   SESSION_QUERY_SESSION_NOT_FOUND: {
     code: 'SESSION_QUERY_SESSION_NOT_FOUND',
     message: 'session was not found',