fix(core): resolve undefined symbols hidden by lib_provisioning star-imports

Six symbols were referenced across the codebase but had no definition anywhere.
Star-imports from lib_provisioning/mod.nu silenced the missing-def errors at
parse time; at runtime the call sites either threw or took dead code paths.
ADR-025 Phase 2 (AST audit) surfaced them as blockers for Phase 3 because
selective imports would expose them as "variable not found" errors.

Resolution: add stub getters in lib_provisioning/config/accessor/functions.nu
following the existing pattern (env -> config -> PROVISIONING-derived -> ""):

  - get-providers-path          (14 call sites)
  - get-prov-lib-path           (2 call sites)
  - get-core-nulib-path         (7 call sites)
  - get-provisioning-generate-dirpath  (5 call sites)
  - get-provisioning-generate-defsfile (1 call site)
  - get-provisioning-req-versions (4 call sites)

All existing callers already guard results with is-empty / path exists checks,
so empty-string returns fall back to safe no-op paths.

show_tools_info (main_provisioning/tools.nu) was missing a guard around its
open call; added is-empty / path-exists check matching sibling fns.

The only non-path symbol (on_clusters in clusters/create.nu) had no recoverable
implementation; its closure is replaced with a user-facing message directing
to 'prvng cluster deploy' (the supported workflow).

Refs: ADR-025, .coder/benchmarks/phase2-findings.md blockers section

nulib/lib_provisioning/mod.nu

@@ -11,7 +11,8 @@ export use context.nu *
 export use setup *
 #export use deploy.nu *
 export use extensions *
-export use providers.nu *
+# providers.nu deleted (ADR-025 blocker 3) — callers import directly from
+# extensions/providers/prov_lib/middleware.nu
 export use workspace *
 export use config *
 export use diagnostics *