fix(agents): stream codex prompts via stdin

Signed-off-by: John Myers <johntmyers@users.noreply.github.com>

Author: johntmyers

scripts/agents/runtime/harnesses/codex/exec.sh

@@ -100,5 +100,5 @@ fi
 exec "$CODEX_BIN" "${CODEX_EXEC_ARGS[@]}" \
     -c "model=\"${CODEX_MODEL}\"" \
     -c "model_reasoning_effort=\"${CODEX_REASONING}\"" \
-    "$(<"$PROMPT_FILE")" \
-    < /dev/null
+    - \
+    < "$PROMPT_FILE"

scripts/agents/runtime/harnesses/codex/subagent.sh

@@ -63,5 +63,5 @@ echo "openshell-agent: invoking Codex sub-agent '$SUBAGENT_ID' (model=$CODEX_MOD
 exec "$CODEX_BIN" "${CODEX_EXEC_ARGS[@]}" \
     -c "model=\"${CODEX_MODEL}\"" \
     -c "model_reasoning_effort=\"${CODEX_REASONING}\"" \
-    "$(<"$PROMPT_FILE")" \
-    < /dev/null
+    - \
+    < "$PROMPT_FILE"