Private /tmp, no display, git over SSH and netlock check fixes

- /tmp: the agent gets its own (<state>/tmp, files older than 7 days are
  removed on start) — the host's /tmp holds sockets that lead out of the
  sandbox (tmux, X11, browsers' singleton sockets, other tools).
  OCSB_TMP=shared keeps the old behaviour. /tmp is mounted before the
  project and state binds so they stay visible if they live under /tmp
- DISPLAY, WAYLAND_DISPLAY, XAUTHORITY are empty in the sandbox and
  ~/.Xauthority is hidden; browsers must run headless
- GIT_SSH_COMMAND uses -F ~/.ssh/config: in the user namespace root-owned
  system ssh configs look owned by nobody and ssh refuses them
- netlock self-test checks that the proxy opens the tunnel, not that a
  remote site answers; failed self-tests now say so instead of exiting
  silently
- doctor: no false error for opencode outside a project
- skill and READMEs: worktrees only for code changes, private /tmp,
  headless browsers, troubleshooting entries

lib/common.sh

@@ -19,7 +19,7 @@ ocsb_info() { echo "→ $*"; }
 
 OCSB_SCALARS=(
   OCSB_NAME OCSB_WEB_PORT OCSB_EGRESS_PORT OCSB_NETLOCK_GROUP OCSB_SANDBOX OCSB_PANEL
-  OCSB_NOTIFICATIONS OCSB_OPENCODE_BIN OCSB_ENV_FILE OCSB_KUBECONFIG OCSB_PASSWORD_FILE
+  OCSB_NOTIFICATIONS OCSB_TMP OCSB_OPENCODE_BIN OCSB_ENV_FILE OCSB_KUBECONFIG OCSB_PASSWORD_FILE
   OCSB_WORKTREES_DIR OCSB_WT_PREFIX OCSB_WT_BASE OCSB_WT_SLUG_REGEX OCSB_DOCKER_GW
   OCSB_STATE_DIR OCSB_OPENCODE_CONFIG OCSB_EGRESS_BASE
 )
@@ -33,6 +33,7 @@ ocsb_defaults() {
   OCSB_SANDBOX=1               # bubblewrap; 0 = egress filter and netlock only
   OCSB_PANEL=1                 # "Tokens" panel in the web UI (plugins/context-panel)
   OCSB_NOTIFICATIONS=1         # desktop notifications through a filtered D-Bus proxy
+  OCSB_TMP=private             # private: own /tmp in the state dir; shared: the host's /tmp
   OCSB_OPENCODE_BIN=""         # default: `opencode` from PATH
   OCSB_ENV_FILE=""             # KEY=VALUE file with tokens, exported to OpenCode
   OCSB_KUBECONFIG=""           # kubeconfig for the agent (~/.kube/config stays hidden)
@@ -53,7 +54,7 @@ ocsb_defaults() {
   OCSB_HIDE_HOME=(.gnupg .aws .azure .config/gcloud .kube/config .docker/config.json .netrc
     .git-credentials .config/gh/hosts.yml .password-store .local/share/keyrings .mozilla
     .config/google-chrome .config/chromium .config/BraveSoftware .claude .codex
-    .config/opencode .local/share/opencode)
+    .config/opencode .local/share/opencode .Xauthority)
   OCSB_SKILLS=(opencode-sandbox)  # skills mounted for the agent (names of skill directories)
   OCSB_SKILL_DIRS=()           # extra directories to look for skills in
   OCSB_PLUGINS=(skill-compact-resurrect/skill-compact-resurrect.v2.ts)  # files in plugins/