Add a native completed-turn hook for bounded memory capture

Kimi hook request: native completed-turn event for bounded memory capture

Summary

Kimi plugins cannot automatically retain the final user/assistant exchange.
Stop exposes stop_hook_active, StopFailure exposes an error, and
SessionEnd exposes a reason, but none provides the final user-visible answer
with stable product turn correlation.

Requested event

Add an observation-only completed-turn event carrying bounded structured data:

{
  "hook_event_name": "TurnCompleted",
  "conversation_key": "product-conversation-id",
  "turn_id": "product-turn-id",
  "prompt": [{"type": "text", "text": "user text"}],
  "response": [{"type": "text", "text": "final assistant text"}],
  "outcome": "completed"
}

The event should also represent cancelled, blocked, and failed outcomes
without fabricating a response. It must fire after the final answer is secured,
not for stream deltas, reasoning, tool requests, or intermediate messages.

Required semantics

• Product conversation_key + turn_id matches the pre-answer identity and is
  stable across callback retry.
• Retried delivery of one event preserves the same identity.
• The final response is exactly the user-visible answer and excludes hidden
  reasoning by contract.
• Payload sizes and content-part types are documented and bounded.
• Hook failure never delays, changes, or erases the completed Kimi answer.
• Private/incognito sessions can disable the event through an app-owned policy.

Use case

A memory plugin can strip its own injected wrappers, apply user-approved
privacy/size rules, and send one idempotent retain operation to an existing
memory authority. Without this native event, alternatives require transcript
database reads, UI scraping, log parsing, or a second permanent client—all less
private and less reliable than an app-owned lifecycle callback.

No files, tests, or entry points are named. Start by tracing the existing Stop, StopFailure, and SessionEnd hook paths and how final answers are secured and correlated. Done means a bounded TurnCompleted event covers each outcome with stable retry identity, documented content limits, privacy policy support, and no impact when delivery fails.