docs: add security policy and vulnerability reporting guidance

SECURITY.md

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+# Security Policy
+
+This policy covers `langgraph-checkpoint-redis`, maintained in
+`redis-developer/langgraph-redis`, including its Redis-backed checkpoint
+savers, stores, serialization, and agent middleware.
+
+## Reporting a Vulnerability
+
+Please report suspected security vulnerabilities privately. Do not include
+vulnerability details or reproduction code in public issues, discussions,
+or pull requests.
+
+You can report a vulnerability through either of these channels:
+
+- **GitHub private vulnerability reporting:** Open the repository’s
+  [Security tab](https://github.com/redis-developer/langgraph-redis/security)
+  and select **Report a vulnerability** to share details privately with
+  the maintainers.
+- **Redis Vulnerability Disclosure Program:** Submit a report through the
+  [Redis disclosure program](https://redis.io/redis-responsible-vulnerability-disclosure/),
+  identifying the repository as
+  `https://github.com/redis-developer/langgraph-redis` and the Python
+  package as `langgraph-checkpoint-redis`.
+
+For questions about reporting or difficulty accessing either channel,
+contact security@redis.com.
+
+### What to Include
+
+Please provide as much of the following as possible:
+
+- A description of the vulnerability and its potential security impact.
+- The affected package version or commit.
+- A minimal reproduction, including code and required configuration.
+- Python, Redis, `redis`, `redisvl`, and `langgraph-checkpoint` versions.
+- The affected component, including whether it uses synchronous,
+  asynchronous, full, or shallow checkpointing.
+- Relevant deployment details, such as standalone Redis, Redis Cluster,
+  or a managed Redis service.
+- The access an attacker needs and which inputs they can control.
+- Any proposed mitigation or fix, if available.
+
+For issues involving checkpoints, stores, or middleware, explain how
+thread identifiers, namespaces, cache configuration, or stored data
+contribute to the issue.
+
+Use synthetic data and remove credentials, connection secrets, personal
+information, and customer data from reproductions.
+
+You do not need to establish the root cause before reporting.
+
+## Handling Reports
+
+Reports are assessed with Redis Product Security and the project
+maintainers. Assessment includes validating the reproduction, identifying
+affected versions and dependencies, and determining impact and severity.
+
+For confirmed vulnerabilities, we coordinate remediation and disclosure
+with the reporter. We may request additional information or help
+validating a fix. Response and remediation times depend on the severity
+and complexity of the issue.
+
+Please coordinate public disclosure with us so that users can receive
+fixes or mitigations before technical details are published.
+
+## Supported Versions and Security Fixes
+
+Security fixes target the latest released version of
+`langgraph-checkpoint-redis`. Users should upgrade to the latest release
+and review its dependency requirements.
+
+Backports to older releases are considered case by case, based on
+severity, compatibility, and maintenance effort. Maintenance of every
+previous release is not guaranteed.
+
+When a security fix is released, the advisory or release notes will
+identify affected versions, fixed versions, and any available mitigations.
+Some fixes may require upgrading dependencies.
+
+## Scope and Deployment Context
+
+We welcome reports involving the confidentiality, integrity, or
+availability of checkpoint data, stored memory, cached responses, and
+tool results. Examples include unsafe deserialization, unintended data
+access or modification, and query or key handling that accesses data
+outside the requested scope.
+
+The library runs within an application and uses that application's Redis
+credentials. Applications are responsible for authenticating callers,
+authorizing access to threads and namespaces, and configuring appropriate
+Redis access controls, network protection, and TLS.
+
+When reporting an issue, describe these application controls and the
+trust boundary you believe the library crosses. If you are unsure whether
+an issue belongs to this library or a dependency, report it privately and
+include the relevant details.