refactor(telemetry): tighten alias helpers and gate on MEM0_TELEMETRY

Review pass on the PostHog identity-stitching fix:

- Gate _maybe_alias_anon_to_email and _maybeAliasAnonToEmail on telemetry
  being enabled. Previous version still read ~/.mem0/config.json and
  persisted telemetry.aliased_to even when MEM0_TELEMETRY=false, which
  meant re-enabling telemetry later would skip identity merging forever.
  Python checks client_telemetry.posthog; TS exports a new
  isTelemetryEnabled() getter from telemetry.ts.
- Trim capture_identify properties to {$anon_distinct_id, client_source}.
  The full platform metadata duplicated capture_event's payload and is
  not needed for $identify; PostHog enriches the person profile from
  subsequent events anyway.
- Flatten config.ts: drop the four-helper structure (loadNodeModules /
  configPath / loadRawConfig / read+write) into one getNodeFs() + a
  shared loadConfig(). Same behaviour, ~30 fewer lines, easier to read.
- Drop the tautological Mem0AnonIds shape test and the
  void createMockFetch hack at the end of the TS test file.
- Add tests for the new telemetry-disabled gate (Python + TS).

Logic traced end-to-end: 114 Python tests + 590 TS tests pass. Ruff and
Prettier clean. Zero new typecheck errors.

mem0-ts/src/client/telemetry.ts

@@ -81,6 +81,10 @@ class UnifiedTelemetry implements TelemetryClient {
   }
 }
 
+function isTelemetryEnabled(): boolean {
+  return MEM0_TELEMETRY;
+}
+
 const telemetry = new UnifiedTelemetry(POSTHOG_API_KEY, POSTHOG_HOST);
 
 async function captureClientEvent(
@@ -110,4 +114,4 @@ async function captureClientEvent(
   );
 }
 
-export { telemetry, captureClientEvent, generateHash };
+export { telemetry, captureClientEvent, generateHash, isTelemetryEnabled };