fix(langgraph): resolve tool bank IDs from config

[koriyoshi2041]

Summary

• allow LangGraph tools to omit a static bank_id and resolve it from RunnableConfig configurable values
• keep static bank_id precedence for existing single-bank usage
• document the multi-user tools pattern and add regression coverage for retain/recall/reflect

Fixes #2442.

Tests

• cd hindsight-integrations/langgraph && uv run pytest tests/test_tools.py
• cd hindsight-integrations/langgraph && uv run pytest tests
• cd hindsight-integrations/langgraph && uvx ruff format --check hindsight_langgraph tests
• cd hindsight-integrations/langgraph && uvx ruff check --config ../../ruff.toml hindsight_langgraph tests
• git diff --check

Note: the repository pre-commit hook could not complete locally because the checkout is missing control-plane eslint dependencies (@eslint/js) and uv run --no-project ruff could not spawn ruff for integrations; the focused checks above passed.