refactor(platform/mod): selective re-exports replace 8 star re-exports (ADR-025 L3)

platform/mod.nu had 8 `export use X *`. Each is now explicit symbol list.

Symbol counts per target:
  target.nu           12
  discovery.nu         5
  health.nu            4
  credentials.nu       6
  connection.nu        9
  cli.nu               7
  autostart.nu         7
  service-manager.nu  15

Total: 65 symbols re-exported.

Pre-existing duplicate symbols surfaced by the explicit listing (not caused
by this commit — the `export use X *` pattern silently shadowed them):
  - get-service-status: connection.nu (arg) + autostart.nu (no arg)
  - start-required-services: service-manager.nu + autostart.nu
  - load-deployment-mode: target.nu + service-manager.nu

Leaving duplicates as-is; resolving the naming collisions is out of scope
for ADR-025 (tracked as pre-existing bug to address in a separate commit).

Validation:
  nu --ide-check 50 platform/mod.nu -> 0 errors

Refs: ADR-025, .coder/benchmarks/phase2-transitivity.md Layer 3

nulib/lib_provisioning/platform/mod.nu

@@ -15,11 +15,42 @@
 # - Service startup management and lifecycle
 # - CLI commands
 
-export use target.nu *
-export use discovery.nu *
-export use health.nu *
-export use credentials.nu *
-export use connection.nu *
-export use cli.nu *
-export use autostart.nu *
-export use service-manager.nu *
+# platform/ subsystem facade — selective re-exports (ADR-025 Phase 3 Layer 3).
+
+export use target.nu [
+    detect-platform-mode get-default-platform-target get-deployment-service-config
+    get-enabled-services get-platform-endpoint get-platform-service-config
+    is-platform-service-enabled list-enabled-platform-services
+    list-required-platform-services load-deployment-mode should-start-locally
+    validate-platform-target
+]
+export use discovery.nu [
+    is-service-available list-required-services list-services
+    service-config service-endpoint
+]
+export use health.nu [
+    check-all-services check-required-services check-service-health wait-for-service
+]
+export use credentials.nu [
+    credential-exists delete-credential get-credential get-credentials-namespace
+    list-workspace-credentials store-credential
+]
+export use connection.nu [
+    add-service-connection get-active-connections get-service-status
+    init-connection-metadata load-connection-metadata remove-service-connection
+    show-connection-status store-connection-metadata update-service-status
+]
+export use cli.nu [
+    platform-config platform-connections platform-health platform-init
+    platform-list platform-start platform-status
+]
+export use autostart.nu [
+    disable-autostart enable-autostart get-service-status restart-service
+    start-required-services start-service stop-service
+]
+export use service-manager.nu [
+    get-external-services get-service-port is-port-listening load-deployment-mode
+    load-service-config nats_health nats_start nats_stop ncl-sync-start
+    ncl-sync-status ncl-sync-stop normalize-service-name start-required-services
+    start-services stop-services
+]