Support pluggable session storage backends

[anthonychu]

Problem Statement

The SDK currently persists all session state to a hardcoded filesystem path (~/.copilot/session-state/{sessionId}/). While this works well for single-node and local development scenarios, it creates significant challenges for production deployments that need:

1. Horizontally scaled services: Multiple server instances can't share session state without a shared filesystem or sticky sessions.
2. Serverless / ephemeral compute: Environments like Azure Functions, or short-lived containers have no durable local filesystem at all.
3. Custom retention and compliance: Organizations may need session data stored in specific databases or regions for audit, TTL, or data residency requirements.
4. Operational consistency: Teams already running Redis, PostgreSQL, etc for other application state want to use the same infrastructure for session data rather than bolting on a filesystem dependency.

Currently this can be achieved by mounting a share as described in ~/.copilot/session-state/ to a shared volume (as described in the Session Persistence guide). It works but this isn't practical or possible in all situations.

Proposed Solution

Introduce a session store abstraction in SDKs to allow saving and rehydrating sessions.

[github-actions]

Thank you for opening this issue, @anthonychu!

I've labeled this as an enhancement. This is a well-articulated feature request for pluggable session storage backends to address:

• Horizontal scaling scenarios
• Serverless/ephemeral compute environments
• Custom retention and compliance requirements
• Operational consistency with existing infrastructure

The current filesystem-only approach (~/.copilot/session-state/) does create limitations for these production deployment patterns. While mounting shared volumes works in some cases, a storage abstraction would provide much more flexibility.

This request is related to but distinct from #386 (which addressed CLI server in-memory state) — this focuses on the SDK's persistent session state storage layer.

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