opencode-sandbox: OpenCode 2 in a sandbox for any project

Runs OpenCode 2 with full permissions while containing its mistakes:

- egress filter: squid in a container per project; squid.conf is rendered
  from layered egress.conf files (package, user, project, local) with
  allow/block/connect rules and expect-pass/expect-block self-tests
- netlock: nftables rejects direct egress for the opencode-sandbox group
  (installed once with sudo as a systemd unit)
- bubblewrap: read-only host, writable project, worktrees and state;
  credentials hidden; ssh keys via the host's ssh-agent; session bus
  replaced by an xdg-dbus-proxy limited to notifications; no permission
  prompts (sudo and systemd-run denied); refuses to run unsandboxed
- per-project state in ~/.local/share/opencode-sandbox/<name>
- worktrees: `wt add|rm|list` with project hooks
- skills mounted per project; the opencode-sandbox skill explains the
  environment to the agent
- plugins as submodules: context-panel, skill-compact-resurrect;
  global config adds opencode-notifier and cc-safety-net
- README in English and Russian

shim/xdg-open

@@ -0,0 +1,8 @@
+#!/bin/sh
+# xdg-open for processes inside opencode-sandbox (OpenCode and the agent's shell). A browser
+# started from here would inherit the netlock group and the proxy, and an already running
+# one would open the agent's link without any filter. opencode-sandbox opens the web UI
+# itself, from the user's normal group.
+echo "xdg-open is disabled in the sandbox: $*" >&2
+echo "Ask the user to open the link." >&2
+exit 1