feat: add OSC 52 clipboard support for container-to-host copy

Programs inside the container (e.g. Claude Code's "hit c to copy") can
now write to the host system clipboard. A shell script shim installed as
xclip/xsel/pbcopy emits OSC 52 escape sequences, which the xterm.js
frontend intercepts and forwards to navigator.clipboard.writeText().

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

container/Dockerfile

@@ -101,6 +101,16 @@ WORKDIR /workspace
 
 # ── Switch back to root for entrypoint (handles UID/GID remapping) ─────────
 USER root
+
+# ── OSC 52 clipboard support ─────────────────────────────────────────────
+# Provides xclip/xsel/pbcopy shims that emit OSC 52 escape sequences,
+# allowing programs inside the container to copy to the host clipboard.
+COPY osc52-clipboard /usr/local/bin/osc52-clipboard
+RUN chmod +x /usr/local/bin/osc52-clipboard \
+    && ln -sf /usr/local/bin/osc52-clipboard /usr/local/bin/xclip \
+    && ln -sf /usr/local/bin/osc52-clipboard /usr/local/bin/xsel \
+    && ln -sf /usr/local/bin/osc52-clipboard /usr/local/bin/pbcopy
+
 COPY entrypoint.sh /usr/local/bin/entrypoint.sh
 RUN chmod +x /usr/local/bin/entrypoint.sh
 COPY triple-c-scheduler /usr/local/bin/triple-c-scheduler

container/osc52-clipboard

@@ -0,0 +1,26 @@
+#!/bin/bash
+# OSC 52 clipboard provider — sends clipboard data to the host system clipboard
+# via OSC 52 terminal escape sequences. Installed as xclip/xsel/pbcopy so that
+# programs inside the container (e.g. Claude Code) can copy to clipboard.
+#
+# Supports common invocations:
+#   echo "text" | xclip -selection clipboard
+#   echo "text" | xsel --clipboard --input
+#   echo "text" | pbcopy
+#
+# Paste/output requests exit silently (not supported via OSC 52).
+
+# Detect paste/output mode — exit silently since we can't read the host clipboard
+for arg in "$@"; do
+    case "$arg" in
+        -o|--output) exit 0 ;;
+    esac
+done
+
+# Read all input from stdin
+data=$(cat)
+[ -z "$data" ] && exit 0
+
+# Base64 encode and write OSC 52 escape sequence to the controlling terminal
+encoded=$(printf '%s' "$data" | base64 | tr -d '\n')
+printf '\033]52;c;%s\a' "$encoded" > /dev/tty 2>/dev/null