refactor(config/loader/mod): selective re-exports replace 5 star re-exports (ADR-025 L3)

config/loader/mod.nu had 5 `export use X *`. Each is now explicit.

Symbol counts per target:
  core.nu         1
  validator.nu    6
  environment.nu  4
  test.nu         2
  dag.nu          1

Total: 14 symbols.

With loader/mod.nu now star-free, the orchestrator loader.nu (1-line
re-export) could be converted from `export use loader.nu *` to selective
in config/mod.nu. Tracked as follow-up — once loader/mod.nu and
accessor/mod.nu are both clean, config/mod.nu's 2 orchestrator exceptions
can be resolved.

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

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

nulib/lib_provisioning/config/loader/mod.nu

@@ -2,17 +2,25 @@
 # Purpose: Centralized configuration loading with hierarchical sources, validation, and environment management.
 # Dependencies: interpolators, validators, context_manager, sops_handler, cache modules
 
+# config/loader/ subsystem facade — selective re-exports (ADR-025 Phase 3 Layer 3).
+
 # Core loading functionality
-export use ./core.nu *
+export use ./core.nu [load-provisioning-config]
 
 # Configuration validation
-export use ./validator.nu *
+export use ./validator.nu [
+    validate-config validate-config-structure validate-data-types
+    validate-file-existence validate-path-values validate-semantic-rules
+]
 
 # Environment detection and management
-export use ./environment.nu *
+export use ./environment.nu [
+    apply-environment-variable-overrides detect-current-environment
+    get-available-environments validate-environment
+]
 
 # Testing and interpolation utilities
-export use ./test.nu *
+export use ./test.nu [create-interpolation-test-suite test-interpolation]
 
 # DAG config accessor (execution, resolution, events defaults merged with workspace dag.ncl)
-export use ./dag.nu *
+export use ./dag.nu [get-dag-config]