Add Agent Assistant continuation and session bridges to turn-kit

[khaliqgant]

Summary

The turn-kit foundation in #299 handles a bounded request/response turn and allows callers to override sessionId when a broader session has already been resolved. It does not yet expose reusable continuation or session adapters for clarification, approval, deferred work, or cross-surface continuity.

Add optional bridges to @agent-assistant/continuation and @agent-assistant/sessions. Cloud should remain the owner of durable continuation/session storage and resume delivery; turn-kit should define the portable lifecycle and adapter seams.

Scope

• Add optional turn-kit entry points for session resolution and resumable outcomes.
• Resolve a stable session from assistant, user, transport, conversation, and thread identity.
• Represent needs_clarification, awaiting_approval, and deferred outcomes without treating them as completed turns.
• Persist and resume through injected adapters compatible with the cloud continuation/session stores.
• Validate resume triggers against the original assistant, user, conversation, and expected continuation state.
• Define expiry, cancellation, duplicate-resume, and already-completed behavior.
• Integrate resumed history/context with the existing Agent Assistant turn-context bridge.
• Keep the base turn runner usable without either optional peer dependency.

Acceptance criteria

• A turn can pause for clarification or approval and resume exactly once from a validated reply.
• Expired, mismatched, duplicated, and cross-conversation resume attempts fail closed.
• Separate users and transport conversations cannot resolve to the same session accidentally.
• Cloud adapters can provide durable storage without turn-kit reimplementing database ownership.
• Tests cover same-surface resume, cross-surface session affinity, expiry, duplicate delivery, and identity mismatch.
• Documentation explains when to use conversation memory, sessions, and continuations.