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/exec_probe_i.py

@@ -0,0 +1,54 @@
+#!/usr/bin/env python3
+"""How many concurrent INTERACTIVE (`docker exec -i`) sessions does this host take?
+
+Trivial non-interactive execs succeed 8-way, but the Phew SLA checker's 5-way
+interactive test failed with
+
+    failed to open stdin fifo: error creating fifo ... -stdin: no such file
+    No such exec instance: <id>
+
+which is a containerd exec-session limit, not a checker bug. Interactive execs
+allocate a fifo + a tracked exec instance, so they hit a ceiling that plain
+`docker exec <c> true` never approaches.
+
+    python3 panel/exec_probe_i.py [container] [N]
+"""
+import subprocess
+import sys
+import time
+from concurrent.futures import ThreadPoolExecutor
+
+CONT = sys.argv[1] if len(sys.argv) > 1 else "phew_container_team1"
+N = int(sys.argv[2]) if len(sys.argv) > 2 else 8
+
+
+def interactive(i: int) -> tuple[int, str]:
+    """Mimic the checker: `exec -i`, write a line, read the reply, exit."""
+    p = subprocess.Popen(
+        ["docker", "exec", "-i", CONT, "sh", "-c",
+         "echo $$; read line; echo \"got:$line\""],
+        stdin=subprocess.PIPE, stdout=subprocess.PIPE,
+        stderr=subprocess.STDOUT, text=True, bufsize=0)
+    try:
+        out, _ = p.communicate("hello\n", timeout=45)
+    except subprocess.TimeoutExpired:
+        p.kill()
+        out = "TIMEOUT"
+    return p.returncode, (out or "").strip().replace("\n", " | ")[:150]
+
+
+def main() -> None:
+    print(f"container={CONT}  N={N} interactive execs")
+    ok = 0
+    with ThreadPoolExecutor(max_workers=N) as ex:
+        for rc, out in ex.map(interactive, range(N)):
+            if rc == 0 and "got:hello" in out:
+                ok += 1
+            else:
+                print(f"  FAIL rc={rc}: {out}")
+    print(f"  {ok}/{N} interactive execs OK")
+    print("RESULT:", "PASS" if ok == N else f"limit reached at {ok}/{N}")
+
+
+if __name__ == "__main__":
+    main()