Add unit tests to reach 63.3% coverage (task 335)

- internal/shell/shell_internal_test.go (new): tests prefixCompleter.Do
  with table-driven cases covering empty prefix, partial match, no match,
  and multi-word line (prefix after space)
- internal/cli/cli_test.go (new): tests pure helpers (logMsg, warn,
  printHelp, shredFile), dispatch paths with real store (ls, search, cat,
  rm, shred, get, paste, export, pathexport, open, edit, unknown command,
  empty argv), error paths for cmdAdd/cmdImport/cmdImportR, readPIN env-var
  path, completionFn commands-only path, makeActionFn nil cases
- internal/store/store_test.go: added TestSearchActionPathExport,
  TestSearchActionWithCallback, TestSearchActionNilCallback, TestFzfEmpty,
  TestRemoveInteractiveInvalidThenDecline, TestImportForceOverwrite

Total coverage: 44.8% → 63.3%

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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