v0.5.1

## [0.5.1] — 2026-07-02

Implements AHP 0.5.1.

### Added

- Optional `nonce` field on `ContentRef`.
- `SubscribeParams.delivery.maxLatencyMs` and
  `AHPClient.subscribe(_:delivery:)` for clients to request a maximum
  subscription delivery latency, including `0` for no intentional coalescing.
- Optional `capabilities` field on `AgentInfo` (`AgentCapabilities` with a
  nested `multipleChats` capability carrying `fork`) so clients gate multi-chat
  and fork via advertised capabilities instead of provider-id switches.
- Cursor-based pagination for `listSessions`, via new shared `PaginatedParams`
  (`limit` + `cursor`) and `PaginatedResult` (`nextCursor`) types:
  `ListSessionsParams` and `ListSessionsResult` now carry these fields, letting
  clients page through a large session catalogue. Fully additive — omitting the
  fields preserves prior behaviour.
- `SubscribeParams.view.turns`, `ChatState.turnsNextCursor`, and the
  `chat/turnsLoaded` action so clients can subscribe to a bounded tail of chat
  history and page older turns into the reduced chat state on demand.
- `SessionState.inputNeeded` — a session-level aggregate of outstanding input
  requests across all chats (`SessionInputRequest` enum with
  `SessionChatInputRequest`, `SessionToolConfirmationRequest`, and
  `SessionToolClientExecutionRequest` cases), plus the
  `StateAction.sessionInputNeededSet` / `StateAction.sessionInputNeededRemoved`
  actions and the `ToolCallConfirmationState` union. The session reducer
  maintains the `SessionStatus.inputNeeded` activity bit from the queue,
  clearing it (falling back to `.inProgress`) when the last entry is removed.
- Optional `intention` field on `ChatToolCallStartAction` and every tool-call
  lifecycle state.
- Optional `model` and `tools` fields on `AgentCustomization` for a custom
  agent's pinned model and tool allowlist.

### Changed

- `fetchTurns` now accepts `cursor` from `ChatState.turnsNextCursor` and returns
  an empty result after the host has loaded older turns into chat state, instead
  of returning a detached `{ turns, hasMore }` page.
- Generated clients now advertise only protocol `0.5.1`, since the `fetchTurns`
  contract is not wire-compatible with `0.5.0`.

### Removed

- `filter` field from `ListSessionsParams`. It was an untyped placeholder with
  no defined semantics; it will return with a concrete shape once session
  filtering/sorting is specified.

### Fixed

- `SnapshotState` now decodes the `chat` variant. Its decoder previously never
  attempted `ChatState`, so chat snapshots failed to decode. Variant
  disambiguation also no longer relies on the removed `summary` field (a leftover
  from before `SessionState` was flattened).