Wipe memguard buffers on every exit and restore echo (closes #35)

Entry() now returns the exit code and only main calls os.Exit, so the
deferred memguard.Purge() in Entry() runs on success and on error;
before, os.Exit(1) skipped every deferred Destroy().

SIGINT and SIGTERM go through memguard's handler, which wipes every
buffer and exits with status 1. The passphrase prompt turns terminal
echo off until its read returns, and the handler exits before that, so
the handler first restores the terminal settings saved at startup, but
only when this process is in the terminal's foreground process group: a
background process that changes the terminal is stopped instead of
exiting.

Model: opus-5-5

TODO.md

@@ -25,6 +25,13 @@ Bring the repo into policy compliance in one commit:
 
 # Completed Steps
 
+- 2026-10-03: Key material is wiped on every exit: `Entry()` returns
+  the exit code after its deferred `memguard.Purge()` has run, and only
+  `main` calls `os.Exit`. SIGINT and SIGTERM go through memguard's
+  handler, which wipes every buffer before exiting; when the process is
+  in the terminal's foreground process group it first restores the
+  terminal settings from startup, so an interrupted passphrase prompt no
+  longer leaves echo off.
 - 2026-10-02: A plain `docker build .` builds again: the size tests
   skip a case that needs more locked memory than the process can
   lock, and run every case under `script/cibuild`. The image stamps the