fix: feat: SessionEnd hook for guaranteed cleanup on session exit (#276)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/hooks/on-session-end.sh

@@ -0,0 +1,22 @@
+#!/bin/bash
+# on-session-end.sh — SessionEnd hook for dark-factory agent sessions.
+#
+# Called by Claude Code when a session terminates (clean exit, logout,
+# crash, OOM, etc.). Writes a termination marker so monitor_phase_loop
+# can detect session death faster than tmux has-session polling alone.
+#
+# Usage (in .claude/settings.json):
+#   {"type": "command", "command": "this-script /tmp/claude-exited-SESSION.ts"}
+#
+# Args: $1 = marker file path
+
+input=$(cat)  # consume hook JSON from stdin
+
+reason=$(printf '%s' "$input" | jq -r '
+  .matched_hook // .reason // .type // "unknown"
+' 2>/dev/null)
+[ -z "$reason" ] && reason="unknown"
+
+if [ -n "${1:-}" ]; then
+  printf '%s %s\n' "$(date +%s)" "$reason" > "$1"
+fi