agent-host-chat-contributions

Create a directory under node/chatContributions/, implement IAgentHostChatContribution with a unique static id and explicit order, inject services through the constructor after IAgentHostChatContributionContext, and register it once in builtInChatContributions.ts.

Keep code in AgentSideEffects when it is routing fabric like subagent signal routing or turn-id remapping, a correctness invariant, or provider-shaped mapping such as SDK event mappers and attachment serializers. Convenience is not a valid reason.

onIncomingRequest is a synchronous admission gate, so a throw rejects the request with internalError rather than being skipped. Failing open would let requests bypass guards like the read-only or worktree checks, while observer hooks only lose one enrichment.

Only when no existing payload can express the behavior because it fires at a different moment or from a different source. Prefer adding a discriminant or payload variant to an existing type, such as a new TurnEndReason, over a single-purpose hook.

Chat mementos are evicted when their chat is disposed and session mementos when their session is disposed. For keys with extra segments, call deleteMemento when the value is no longer needed, since setting undefined leaves the map entry alive until disposal.

Some hand-built test graphs construct AgentSideEffects without calling registerBuiltInChatContributions, so extracted behavior silently stops running. Add the missing registration to mirror production wiring rather than weakening the assertion.