fix(access): ban guest-agent as access channel — SSH only [#403]

Previous sessions used `qm guest exec` to back-door SSH keys into ~30 VMs,
bypassing sshd's audit trail in an ITAR/CMMC environment. Wire the ban deep
so it cannot recur:

- tests/remote.sh: remove the vm-guest mode + qm-guest-exec path entirely
- scripts/check-rules.sh: rule #11 fails on any `qm guest exec` / `vm-guest`
  pattern in code (scans .sh/.bash/.py; docs may describe the ban freely)
- AGENTS.md: codify "Access-channel policy: SSH only" as non-negotiable;
  add "Questions" rule banning harness question tools (use questions-v1.md)
- tests/vm-validation.sh: drop guest-agent key re-injection; SSH-only
- proxmox/perf/scripts/perf-matrix.sh + deploy-tuned-guests.sh: convert
  guest-agent execution to SSH (vmroot) now that VMs have key + sudo
- bootstrap-all.sh: re-target the 8 remaining locked-out systems with
  correct users/methods; print a console one-liner for publickey-only Pis

Guest-agent remains installable/checkable for Proxmox state visibility —
never as an execution or key-delivery path.

💘 Generated with Crush

Assisted-by: Crush:glm-5.2

scripts/check-rules.sh

@@ -242,4 +242,25 @@ if [ "$RULE_FAST" = false ] && [ -x scripts/test.sh ]; then
     fi
 fi
 
+# ----------------------------------------------------------------------------
+# 11. Access-channel policy — qemu guest-agent must NEVER be used as an
+#     execution or key-delivery channel. SSH (sshd) is the ONLY approved
+#     remote access path; all commands must be logged through standard
+#     auth/audit infrastructure. ITAR/CMMC environment — non-negotiable.
+#     Allowed: installing/checking qemu-guest-agent for Proxmox state
+#     visibility. Forbidden: `qm guest exec` + any `vm-guest` wrapper.
+#     Scans CODE only (not .md) so docs may describe the ban.
+# ----------------------------------------------------------------------------
+$RULE_VERBOSE && log_step "Access-channel policy (no guest-agent exec)"
+GA_HITS="$(grep -rnE 'qm guest exec|vm-guest|_vm_guest' \
+    --include='*.sh' --include='*.bash' --include='*.py' \
+    . 2>/dev/null | grep -vE 'scripts/check-rules\.sh|/archive/' || true)"
+if [ -z "$GA_HITS" ]; then
+    check "No guest-agent exec / vm-guest access patterns in code" "pass"
+else
+    GA_COUNT="$(printf '%s\n' "$GA_HITS" | grep -c . || true)"
+    $RULE_VERBOSE && printf '%s\n' "$GA_HITS" | sed 's/^/    /'
+    check "${GA_COUNT} guest-agent exec / vm-guest reference(s) — SSH-only access policy (AGENTS.md)" "fail"
+fi
+
 print_summary_and_exit