fix(sla): 100% fleet SLA (64/64) - per-challenge SSH login, phew checker, sidecar detection

Three independent root causes, all found by measuring instead of assuming:

1. SSH failed on 10/16 challenges while state.json looked perfect.
   Only the 6 native GEMASTIK XVIII images provision 'ctfuser'; every imported
   XVI/XVII image does 'echo root:${PASSWORD} | chpasswd' and logs in as root.
   set_ssh_passwords() hardcoded ctfuser, so chpasswd set a password nobody
   used -> 'Permission denied' on every team. Registry gains a per-challenge
   'ssh_user'; chpasswd targets the real login and reports failures loudly.

2. phew SLA timed out on a healthy service, four bugs stacked:
   - chall.py block-buffers stdout through the exec pipe (PYTHONUNBUFFERED now
     set) and does a fresh Pailier keygen (~12 s) before printing its menu;
   - _read_until read a TEXT pipe, so read(1) pulled 8 KB into Python's
     TextIOWrapper buffer and select() then blocked on data already in memory;
   - its buffer was per-call, so the read satisfying 'pt (hex)' also swallowed
     the '> ' the next call waited for -> a race that failed intermittently;
   - reaping killed chall.py it did not own: a blanket pkill -f, a
     snapshot-diff (concurrent sessions diff against the same pre-spawn set),
     and a class-level _children shared across uvicorn's thread pool. The child
     now prints its own pid so exactly one session is reaped.
   Also: ONE interactive session per check instead of five spawns (Paillier is
   randomized per ciphertext, not per process) - 5 keygens were the CPU load
   that starved the checks. And the 6 orphan single-node containers from the
   original deploy were removed; one held 58 leaked chall.py and drove load
   average 76 on 2 CPUs.

3. missing_sidecars() matched compose-generated names (teamN-<svc>-1) while
   every service sets an explicit container_name, so it reported all 16 running
   challenges as missing and hid the one real gap (anti-alchemy-db, which has
   no container_name). Now reads container_name when present and falls back to
   the compose default otherwise.

Verified: 64/64 SLA across 4 teams; 64/64 real SSH logins succeed with
correct <chall>_teamN hostnames; phew 3/3 sequential with no process leak.

Adds panel/verify_ssh_creds.py, audit_ssh_users.sh, reset_runtime.sh,
sla_sweep.sh, fix_sidecars.sh, phew_concurrency_test.sh, exec_probe_i.py.

panel/phew_concurrency_test.sh

@@ -0,0 +1,35 @@
+#!/usr/bin/env bash
+# Concurrency regression test for the Phew checker.
+#
+# The bug this guards against: the checker reaped chall.py processes it did not
+# own, so two overlapping checks killed each other's session and the victim
+# reported "Process ended while waiting for '> '" on a healthy service. Firing
+# several checks at once is the only way to reproduce it — sequential runs pass
+# even with the bug present.
+set -uo pipefail
+BASE=/opt/gemastik18-final
+TEAM=${1:-1}
+N=${2:-5}
+case "$TEAM" in 1) PORT=31080;; 2) PORT=32080;; 3) PORT=33080;; 4) PORT=34080;; esac
+U=$(grep -oP '^ADMIN_USERNAME=\K.*' "$BASE/teams/team$TEAM/receiver/.env")
+P=$(grep -oP '^ADMIN_PASSWORD=\K.*' "$BASE/teams/team$TEAM/receiver/.env")
+
+count() { docker exec "phew_container_team$TEAM" sh -c 'ps ax | grep -c "[c]hall.py"'; }
+echo "before: $(count) chall.py (1 = socat service only)"
+
+pids=()
+for i in $(seq 1 "$N"); do
+  ( out=$(timeout 300 curl -sS -u "$U:$P" "http://127.0.0.1:$PORT/check/phew")
+    echo "  req$i: $out" ) &
+  pids+=($!)
+done
+for p in "${pids[@]}"; do wait "$p"; done
+
+sleep 5
+after=$(count)
+echo "after:  $after chall.py"
+if [ "$after" -le 1 ]; then
+  echo "RESULT: PASS (no leak)"
+else
+  echo "RESULT: FAIL (leaked $((after - 1)))"
+fi