chore: fix all build warnings and add stabilization roadmap

- Fix 3× ResultShadowed in cypher.nim, recovery.nim, shell.nim
- Remove 6× UnusedImport in test files
- Build is now clean: 0 warnings, 294 tests pass
- Add 4-week stabilization plan to PLAN.md (Sessions 9.1–9.4)

tests/test_all.nim

@@ -4,8 +4,6 @@ import std/tables
 import std/strutils
 import std/os
 import std/asyncdispatch
-import std/times
-import std/random
 import std/monotimes
 import std/base64