agent-host-chat-contributions

Create a new directory under `node/chatContributions/<feature>/` with a class implementing `IAgentHostChatContribution`. Give it a unique static id, an explicit order, and a constructor whose first parameter is `IAgentHostChatContributionContext`. Register it once in `builtInChatContributions.ts`.

Contributions react to lifecycle moments through named hooks and can be dependency-injected without changing protocol routing. Routing fabric like subagent signal buffering and turn-id remapping must stay in `AgentSideEffects` because it routes every signal correctly before state changes.

Only add a hook when no existing payload can express the behavior because it fires at a different moment or from a different source. Prefer adding a discriminant like a new `TurnEndReason` variant over introducing a single-purpose hook with one caller.

`onIncomingRequest` is the admission gate that enforces the read-only guard and hides the composer in the UI. A throwing contribution there rejects the request with `internalError` at the validation stage, preventing work in sessions that may have lost their isolated worktree.

Use `createChatMementoKey` or `createSessionMementoKey` from `agentHostChatContributionsService.ts`. Call `deleteMemento` when an extra-segment value is no longer needed, because setting its observable to undefined leaves the map entry alive until chat or session disposal.

Lower order values run first, with registration order breaking ties. LocalCommand is 50 and TurnAdmission is 100 on `onIncomingRequest` so local commands intercept before the read-only guard. Update the built-in-sequence regression tests in `chatContributions.test.ts` when changing order intentionally.