AgentBBS: M0 core hub, M1 arcade, pods with CoinPay membership

A modern BBS over SSH for humans and AI agents (docs/PRD.md), plus the
pods addendum (docs/pods.md). Go + charmbracelet (wish/bubbletea).

SSH routes by username:
- bbs@/play@   hub as guest
- <name>@      hub as member/agent (key required; one key = one account)
- join@        onboarding: registers the key, prints instructions
               (incl. coinpay pay command with HMAC payment ref), kicks
- pod@         personal Linux container, paid membership $1/mo via
               CoinPay; rootless podman preferred, hardened docker
               fallback (cap-drop ALL, no-new-privileges, uid 1000,
               cpu/mem/pids caps, per-user volume)

M0: plugin contract (ID/Title/Description/RequiresAuth/New + ExitMsg),
hub menu, SQLite store (users/sessions/scores/pod_subscriptions),
session audit, grant-pod ops command.

M1 arcade: doom-ascii + Freedoom via scripts/fetch-assets.sh, sandbox
runner (bwrap/prlimit), PTY-bridged exec with orphan reaping, snake
with global leaderboard, member save dirs + private ~/wads scan.

Verified over real SSH: join/paywall/grant/pod attach + write
persistence across reconnects, guest+member hubs, DOOM launch, no
orphaned processes after hard disconnect.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/sandbox/sandbox.go

@@ -0,0 +1,105 @@
+// Package sandbox wraps game/agent subprocesses with per-session isolation
+// and resource limits (PRD §7 S2). It prefers bubblewrap, falls back to
+// prlimit, and degrades to a plain exec with a warning.
+package sandbox
+
+import (
+	"fmt"
+	"os/exec"
+)
+
+// Mode selects the isolation technology.
+type Mode string
+
+const (
+	ModeAuto    Mode = "auto"
+	ModeBwrap   Mode = "bwrap"
+	ModePrlimit Mode = "prlimit"
+	ModeNone    Mode = "none"
+)
+
+// Limits are per-process resource caps.
+type Limits struct {
+	CPUSeconds int // hard CPU-time cap (fork-bomb/runaway protection)
+	MemoryMB   int
+	MaxProcs   int
+}
+
+// DefaultLimits suit a single interactive game session.
+var DefaultLimits = Limits{CPUSeconds: 3600, MemoryMB: 512, MaxProcs: 64}
+
+// Runner builds sandboxed exec.Cmds.
+type Runner struct {
+	mode Mode
+}
+
+// New picks the best available mode when ModeAuto is requested.
+func New(mode Mode) *Runner {
+	if mode == "" || mode == ModeAuto {
+		switch {
+		case have("bwrap"):
+			mode = ModeBwrap
+		case have("prlimit"):
+			mode = ModePrlimit
+		default:
+			mode = ModeNone
+		}
+	}
+	return &Runner{mode: mode}
+}
+
+// Mode reports the active isolation mode.
+func (r *Runner) Mode() Mode { return r.mode }
+
+func have(bin string) bool { _, err := exec.LookPath(bin); return err == nil }
+
+// Command wraps program+args in the runner's sandbox. workDir is the only
+// writable path (savegames land there); everything else is read-only.
+func (r *Runner) Command(workDir, program string, args ...string) *exec.Cmd {
+	lim := DefaultLimits
+	switch r.mode {
+	case ModeBwrap:
+		bw := []string{
+			"--ro-bind", "/", "/",
+			"--dev", "/dev",
+			"--proc", "/proc",
+			"--tmpfs", "/tmp",
+			"--bind", workDir, workDir,
+			"--unshare-net",
+			"--unshare-pid",
+			"--die-with-parent",
+			"--chdir", workDir,
+		}
+		// Resource caps still come from prlimit when available.
+		if have("prlimit") {
+			pl := prlimitArgs(lim)
+			full := append(pl, "bwrap")
+			full = append(full, bw...)
+			full = append(full, "--", program)
+			full = append(full, args...)
+			return exec.Command("prlimit", full...)
+		}
+		full := append(bw, "--", program)
+		full = append(full, args...)
+		return exec.Command("bwrap", full...)
+	case ModePrlimit:
+		full := append(prlimitArgs(lim), program)
+		full = append(full, args...)
+		cmd := exec.Command("prlimit", full...)
+		cmd.Dir = workDir
+		return cmd
+	default:
+		cmd := exec.Command(program, args...)
+		cmd.Dir = workDir
+		return cmd
+	}
+}
+
+func prlimitArgs(l Limits) []string {
+	return []string{
+		fmt.Sprintf("--cpu=%d", l.CPUSeconds),
+		fmt.Sprintf("--as=%d", l.MemoryMB*1024*1024),
+		fmt.Sprintf("--nproc=%d", l.MaxProcs),
+		"--",
+	}
+}