fix: update AD-002 docs and stale comments to reflect CLAUDE_CONFIG_DIR isolation (#647)

- AGENTS.md AD-002: document per-session CLAUDE_CONFIG_DIR as primary
  OAuth concurrency guard, CLAUDE_EXTERNAL_LOCK as rollback flag
- docker/agents/entrypoint.sh: update stale flock comment
- lib/agent-sdk.sh: move mkdir inside CLAUDE_EXTERNAL_LOCK branch

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

lib/agent-sdk.sh

@@ -137,13 +137,12 @@ agent_run() {
 
   local run_dir="${worktree_dir:-$(pwd)}"
   local lock_file="${HOME}/.claude/session.lock"
-  mkdir -p "$(dirname "$lock_file")"
   local output rc
   log "agent_run: starting (resume=${resume_id:-(new)}, dir=${run_dir})"
-  # Acquire lock separately (flock cannot exec bash functions)
   # External flock is redundant once CLAUDE_CONFIG_DIR rollout is verified (#647).
   # Gate behind CLAUDE_EXTERNAL_LOCK for rollback safety; default off.
   if [ -n "${CLAUDE_EXTERNAL_LOCK:-}" ]; then
+    mkdir -p "$(dirname "$lock_file")"
     output=$(cd "$run_dir" && ( flock -w 600 9 || exit 1; claude_run_with_watchdog claude "${args[@]}" ) 9>"$lock_file" 2>>"$LOGFILE") && rc=0 || rc=$?
   else
     output=$(cd "$run_dir" && claude_run_with_watchdog claude "${args[@]}" 2>>"$LOGFILE") && rc=0 || rc=$?