fix(security): delete the plaintext key export after decrypt-wallet import (W2-1)

Removing wallet encryption z_exportwallet'd every private key in cleartext to
<datadir>/obsidiandecryptexport<ts>, re-imported it, and never deleted it — leaving a
full plaintext dump of every key on disk permanently. The decrypt flow now scrubs
(best-effort in-place zero-overwrite) and removes that file as soon as the
z_importwallet attempt resolves, on both the success and failure paths. Recovery, if
ever needed, remains the encrypted backup (wallet.dat.encrypted.bak), never this file.

Second fix in the wallet-hardening P0-A cluster (docs/wallet-hardening.md). Not
unit-testable (fs I/O in a deep worker lambda); build-clean, ctest 1/1.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/wallet-hardening.md

@@ -112,4 +112,5 @@ Land W7-2 first — it unblocks the rest.
 
 ## Progress log
 
+- **P0-A / W2-1** — ☑ landed: the decrypt-wallet flow now scrubs (best-effort in-place zero-overwrite) and removes the plaintext key export (`obsidiandecryptexport…`) as soon as the `z_importwallet` attempt resolves — success or failure — so a full cleartext dump of every private key is no longer left on disk forever. Recovery remains the encrypted backup (`wallet.dat.encrypted.bak`). `app_security.cpp` (after the import call). Not unit-testable (fs I/O in a deep lambda); build-clean, `ctest` 1/1 (no regression).
 - **P0-A / W7-1** — ☑ landed: `RedactConsoleCommand`/`ConsoleCommandCarriesSecret` in `console_tab_helpers` redact secret-bearing commands (an allowlist of 13 first-tokens: `walletpassphrase`, `encryptwallet`, `z_importkey`, …) to `> walletpassphrase ****` before they hit the console echo AND the recall history; the real command still executes unredacted. Wired into `submitConsoleCommand` (`console_tab.cpp`). New `testConsoleSecretRedaction` (11 assertions). Clean build; `ctest` 1/1. (Output-secret commands like `z_exportkey` — result redaction — remain a follow-up.)