clients/go/v0.4.0

## [0.4.0] — 2026-06-19

Implements AHP 0.4.0.

### Added

- `MessageOrigin` struct and `MessageKind` type now model `Message.Origin`
  (previously an untyped `json.RawMessage`); `MessageKind` covers `user`,
  `agent`, `tool`, and `systemNotification`, adding faithful agent- and
  tool-initiated origins.
- `ConfigPropertySchema.AdditionalProperties` — optional field describing the
  schema for object-typed config properties beyond those in `Properties`.
- `ChangesetContentChangedAction` for full-replacement changeset file
  snapshots with optional operations and error details.
- `ChangesetOperationStatusDisabled` — new `ChangesetOperationStatus` value for
  operations that are currently unavailable and cannot be invoked.
- `ChangesetOperation.Group` — optional identifier for grouping related
  changeset operations together in the UI.
- `Meta` (wire `_meta`) field on the per-turn chat actions (`chat/turnStarted`,
  `chat/delta`, `chat/responsePart`, `chat/reasoning`, `chat/usage`,
  `chat/turnComplete`, `chat/turnCancelled`, `chat/error`) — optional
  provider-specific metadata so hosts can carry portable per-event context,
  such as attributing an event to a specific agent (e.g. a sub-agent acting
  within the turn).

### Changed

- `ToolResultSubagentContent.Resource` is now specified as the spawned worker
  **chat** URI (`ahp-chat:/<cid>`), not a session URI — a tool-spawned
  sub-agent is a chat. Its doc now describes the correspondence with the worker
  chat's `ChatToolOrigin` record (matching `ToolCallId`), which remains the
  canonical representation of the spawn relationship.
- **BREAKING:** `ChangesetOperationTargetRange` is now a nested `TextRange`
  (`{start: {line, character}, end: {line, character}}`) instead of flat
  `{start, end}` `int64` fields.

### Fixed

- `ActionEnvelope.Origin` is now omitted from JSON output when absent
  (`json:"origin,omitempty"`) instead of serializing as `null`.

### Added

- `ApplyActionToChangeset`, `ApplyActionToAnnotations`, and `ApplyActionToResourceWatch` — full reducer implementations replacing the previous stubs; all shared conformance fixtures in `types/test-cases/reducers/` for these three families now pass.
- `SnapshotState.ResourceWatch` pointer field — the `Snapshot.state` union
  now accepts `ResourceWatchState`, decoded by probing for the required
  `root` + `recursive` keys (ordered between the existing changeset and
  annotations probes).

### Fixed

- Reducer parity fixtures now require `_meta` updates from every
  tool-call-scoped action, not only `session/toolCallStart`.

### Added

- `AnnotationsUpdatedAction` (`annotations/updated`) — partially updates an
  existing annotation's `TurnID` / `Resource` / `Range` / `Resolved` without
  resending its entries. Handled by the annotations reducer (no-op on unknown
  id).

- `ahp-chat:` channel for per-chat conversation state; `SessionState.chats[]` catalog; `SessionState.defaultChat?` input-routing hint; `ChatOrigin` provenance union; `createChat` / `disposeChat` commands.
- `ChatSummary.WorkingDirectory` — optional per-chat working directory. Falls back to the session's `WorkingDirectory` when absent.
- `ChatInteractivity` string enum (`ChatInteractivityFull` / `ChatInteractivityReadOnly` / `ChatInteractivityHidden`) and the optional `ChatSummary.Interactivity` / `ChatState.Interactivity` field describing how the user can interact with a chat. Absent defaults to `"full"`.
- Three discrete chat-catalog actions on the session channel — `SessionChatAddedAction` (upsert by `Summary.Resource`), `SessionChatRemovedAction`, and `SessionChatUpdatedAction` (partial-update payload).
- `SessionDefaultChatChangedAction` (`session/defaultChatChanged`) — updates `SessionState.DefaultChat` to steer new input to the designated chat; absent value clears the hint.
- `ErrorInfo.Meta map[string]json.RawMessage` — optional provider-specific metadata bag on error payloads (`_meta` on the wire), mirroring the existing `Meta` field on `UsageInfo` and other protocol types.
- `RootState` now exposes an optional `_meta` property bag (`Meta
  map[string]json.RawMessage`) for implementation-defined agent-host metadata,
  such as a well-known `hostBuild` key carrying the host's build
  version/commit/date.

### Changed

- `ChatState` is now flat — the previous embedded `Summary` has been replaced with inlined `Resource` / `Title` / `Status` / `Activity` / `ModifiedAt` / `Model` / `Agent` / `Origin` / `WorkingDirectory` fields. `ChatSummary` remains as the standalone catalog entry on `SessionState.Chats`.
- `ChatSummary.ModifiedAt` and `ChatState.ModifiedAt` are now ISO 8601 `string` values instead of integer milliseconds.

### Removed

- `SessionChatsChangedAction` (replaced by the three discrete chat-catalog actions above).