fix: stabilization pass — fail-closed auth, ai flag validation, lint/config/security hardening, regression tests

17-point code-level audit executed via Explorer->Architect->Builder->Tester->Reviewer;
Reviewer accepted (APPROVE_WITH_NOTES; 3 block-list items resolved):

- security: telegram sender-owner AND-gate + TELEGRAM_OWNER_ID, matrix
  MATRIX_ROOM_ID fail-closed, gpg --passphrase-fd 3 (no argv secret),
  /dev/tcp positional-arg form (checkport/smb-client/share-lib/NET_PROBE),
  eval deny-by-default + --no-command-execution carried by both chat bridges,
  tty-gated --trust; config/{telegram,matrix}.env reference templates
- ai: all ExecStart flags validated against installed llama.cpp
  (requested->error, default->omit+warn, CONFIG_REQUESTED_FLAGS); single-file
  hf download failure rc=1 + no .hf-meta; LLAMACPP_HOST coherent;
  POS_SUBCMDS + metadata gaps closed
- tooling: lint-conventions Bash-native rewrite (~24-30x faster, rules and
  output byte-identical, :num restored); pos system uninstall covers all 12
  libs + scale-tail + flags dir + systemd user units (|| true) + plugin
  markers; anchored .bash_completion/.bashrc removal replaces sed -i '/pos/d'
- config: canonical load_env_file in lib/config-ui.sh (CRLF strip, env-wins,
  XDG, LOADED_ENV_KEYS); 9 tools migrated; entertainment-lib collapsed to
  wrappers; docker-compose deliberately unmigrated (source semantics)
- tests: first committed regression suite — tests/run-tests.sh zero-dep
  runner + make test; 12 files / 179 checks / 0 skip / ~52s; hard skip
  contract; systemd-analyze verify on generated unit PASS

Verified: make gen idempotent; make check green; make lint 0 FAIL, 0 WARN;
make test green; bash -n clean; git diff --check clean. Audit deliverables +
agent reports + AGENT_TODO Done entry included.

bin/pos-system-backup

@@ -192,14 +192,22 @@ run_backup() {
         unset CONFIRM
 
         log "Encrypting backup..."
-        gpg --batch --yes --passphrase "$PASS" --symmetric --cipher-algo AES256 "$ARCHIVE"
+        # The passphrase is fed on fd 3 (here-string), never via argv, so it
+        # cannot leak through 'ps' / /proc/<pid>/cmdline while gpg runs.
+        if ! gpg --batch --yes --passphrase-fd 3 --symmetric --cipher-algo AES256 "$ARCHIVE" 3<<<"$PASS"; then
+            rm -f "$ARCHIVE"
+            err "encryption failed — plaintext archive removed, nothing left behind"
+        fi
 
         rm -f "$ARCHIVE"
         ARCHIVE="${ARCHIVE}.gpg"
         chmod 600 "$ARCHIVE"
 
         log "Verifying encrypted backup..."
-        gpg --batch --quiet --passphrase "$PASS" --decrypt "$ARCHIVE" | tar -tzf - > /dev/null
+        if ! gpg --batch --quiet --passphrase-fd 3 --decrypt "$ARCHIVE" 3<<<"$PASS" | tar -tzf - > /dev/null; then
+            rm -f "$ARCHIVE"
+            err "decryption verification failed — corrupt artifact removed"
+        fi
 
         unset PASS
     else