docs: README says why the wallet never clears the clipboard (closes #492)

The owner's ruling on #19 is
now in the README, under ExportPrivKey: copying the key leaves it on the
clipboard, because the clipboard is the user's, clearing it would go
against what they expect, and it could destroy something else they
copied since. ShowRecoveryPhrase copies the phrase the same way and
points back to it. Both describe the warning each password screen
shows on next, which does not mention the clipboard.

Model: opus-5-5

TODO.md

@@ -45,6 +45,14 @@ but the review is broader than any of them.
 
 # Completed Steps
 
+- 2026-10-07: The README says that the wallet never clears the clipboard after
+  the private key or the recovery phrase is copied, and why
+  ([#492](https://git.eeqj.de/sneak/AutistMask/issues/492)): the clipboard is
+  the user's, clearing it would go against what they expect, and it could
+  destroy something else they copied since. It is under ExportPrivKey, with a
+  line under ShowRecoveryPhrase, and names the warning each password screen
+  actually shows, which says nothing about the clipboard.
+
 - 2026-10-07: The Firefox end-to-end suite no longer tolerates any uncaught
   extension error ([#487](https://git.eeqj.de/sneak/AutistMask/issues/487)). Its
   one entry, Firefox reporting a popup promise that settled after the page