feat(story-mode): a helper's own steps, from its transcript

GET /api/sessions/:id/story/helpers/:agentId draws one helper (an Agent call)
as a page from its own transcript, <conversation>/subagents/agent-<id>.jsonl
beside the session's, with the type, description and finished flag from its
.meta.json. The file is derived from the session's own transcript path and an
id checked against [A-Za-z0-9_-]{1,64}; a helper with no file yet is page
null, not an error. The reducer gains a sidechain mode (every helper line is a
sidechain line) and keeps the agentId of Agent results, so the main page links
each helper to its steps. Same gates and ETag/304 as /story, now shared.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/types/story.ts

@@ -82,6 +82,8 @@ export interface StoryHelper {
   result?: string;
   toolUses?: number;
   durationMs?: number;
+  /** Claude Code's id for the helper; its own steps are at `GET /api/sessions/:id/story/helpers/<agentId>`. */
+  agentId?: string;
 }
 
 export interface StoryTests {
@@ -164,6 +166,15 @@ export interface StoryPage extends StoryTurnSummary {
   helpers: number;
 }
 
+/** `GET /api/sessions/:id/story/helpers/:agentId`: one helper's own steps, as one page. */
+export type StoryHelperResponse =
+  | { supported: false; reason: string }
+  | {
+      supported: true;
+      helper: { agentId: string; type?: string; description?: string; finished?: boolean };
+      page: StoryPage | null;
+    };
+
 export type StoryResponse =
   | { supported: false; reason: string }
   | { supported: true; turns: StoryTurnSummary[]; page: StoryPage | null };