docs(audit): add SourceNode column to AuditLog/Notifications/SiteCalls design + plan

- Adds SourceNode varchar(64) NULL to AuditLog, Notifications, and SiteCalls
  tables with role-name semantics: node-a/node-b for site rows (qualified by
  SourceSiteId), central-a/central-b for central direct-write rows.
- New IX_AuditLog_Node_Occurred (SourceNode, OccurredAtUtc) index.
- Reframes CLAUDE.md from documentation-only to implementation project.
- Adds docs/plans/2026-05-23-audit-source-node.md + tasks.json companion.

docs/requirements/Component-SiteCallAudit.md

@@ -36,6 +36,10 @@ Lives in the central MS SQL configuration database — a sibling of the
 
 - **TrackedOperationId** — GUID, primary key. Generated site-side at call time.
 - **SourceSite** — site that issued the call.
+- **SourceNode** — the cluster node on which the call was issued (`node-a` /
+  `node-b`, qualified by `SourceSite`). Nullable. Stamped site-side at submit
+  time and carried verbatim through the combined `CachedCallTelemetry` packet,
+  reconciliation pulls, and the central upsert.
 - **Kind** — `TrackedOperationKind` enum: `ExternalCall` or `DatabaseWrite`.
 - **TargetSummary** — external system + method name for an `ExternalCall`; for a
   `DatabaseWrite`, just the database connection name — intentionally not the SQL