refactor(runtime): remove Eino and make Pi the sole agent engine

Delete the Eino model adapters, callbacks, middleware, orchestration,
summarization and single-agent runner together with their dependencies.
Chat, batch tasks, workflow Agent nodes and sub-agents now share the Pi
runtime, and dispatch is a single code path instead of an engine switch.

Configuration migrates eino_middleware/eino_skills to runtime/skills and
drops the retired callback settings while old files still load. Settings,
i18n and docs describe Pi only. Also carry the Pi follow-up fixes: empty
required lists are omitted from tool schemas, historical tool results stay
with their call batch, model proxies work over HTTP/1.1, single-query
retrieval skips query expansion, and finalization docs match Pi lifecycle
events.

scripts/check-pi-only.sh

@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+set -euo pipefail
+project_root="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+cd "$project_root"
+pi_module_list="$(go list -m all)"
+pi_package_list="$(go list -deps ./...)"
+if grep -E 'github.com/(cloudwego/eino|eino-contrib)' <<< "$pi_module_list
+$pi_package_list"; then
+    echo 'Retired Eino dependency found.' >&2
+    exit 1
+fi
+echo 'Pi-only check passed: no Eino modules or compiled package dependencies.'