feat(apiproxy): host session-log download surface

Streams one ZIP of the root session artifact plus each subagent descendant
verbatim (the persistence readRaw bytes) from GET /api/session.export as a
host-only download — no wire envelope, absent from IApiClient. The downloads
domain owns the query schema, the fetch handler answers the GET alongside the
SSE routes, and compression runs on the host with fflate's streaming Zip API.

packages/host/apiproxy/src/api/index.ts

@@ -16,6 +16,7 @@ import type { GoalsApi } from './goals.ts'
 import type { SettingsApi } from './settings.ts'
 import type { CredentialsApi } from './credentials.ts'
 import type { LlmApi } from './llm.ts'
+import type { DownloadsApi } from './downloads.ts'
 import type { ClientResponse, RpcReceipt } from './rpc.ts'
 
 /** Root interface of the unified API surface. New client-request domain = one new file pair + one field here + one map row. */
@@ -32,6 +33,8 @@ export interface ApiProxy {
   settings: SettingsApi
   credentials: CredentialsApi
   llm: LlmApi
+  /** Host-only download surfaces (GET, no wire envelope); absent from IApiClient. */
+  downloads: DownloadsApi
   /** Response entry for server-requests (client-response, echoing their rpcId); not a domain method (four-quadrant model). */
   respond(message: ClientResponse): Promise<RpcReceipt>
 }
@@ -39,9 +42,8 @@ export interface ApiProxy {
 // ---- Domain interfaces and payload entities ----
 export type {
   HistoryEntry, ModelCatalogFailure, ModelCatalogModel, ModelProviderGroup, ModelReasoning,
-  ModelReasoningEffort, ModelSelection, PromptContentPart, QueueAction, SessionModels, SessionProjectionsBlock,
-  SessionSearchItem,
-  SessionsApi, SessionSummary,
+  ModelReasoningEffort, ModelSelection, PromptContentPart, QueueAction, SessionModels,
+  SessionProjectionsBlock, SessionSearchItem, SessionsApi, SessionSummary,
 } from './sessions.ts'
 export type { DirectoryEntry, DirectoryListing, HostApi } from './host.ts'
 export type {
@@ -57,6 +59,7 @@ export type { GoalsApi, GoalId, GoalRef } from './goals.ts'
 export type { SettingsApi, SettingsNamespaceView, SettingsPathOpView, SettingsSecretView } from './settings.ts'
 export type { CredentialsApi, CredentialView } from './credentials.ts'
 export type { ConfigurableProviderView, DiscoveredModelView, LlmApi } from './llm.ts'
+export type { DownloadsApi } from './downloads.ts'
 export type { ApprovalResponsePayload } from './approvals.ts'
 
 export type { QuestionResponsePayload } from './questions.ts'