feat(tool-youtube-transcript): YouTube transcription tool

New standard tool extension backed by a self-hosted transcriber service
(http://100.102.55.49:41090, Tailscale, no API key). One tool
youtube_transcript — fetches transcripts for YouTube videos. Returns
completed (full text + timestamped segments), queued/processing (position
+ ETA + .youtube_subtitles_pending retry convention), or failed (error).

Pure core: validateUrl + format* functions + truncateOutput. Injected
edge: TranscriptClient (injectable fetchFn, AbortSignal.any for
cancellation). concurrencySafe true, capabilities network. 30 tests.

Verified: tsc EXIT 0, 1152 vitest, biome clean (327 files). Boot smoke
clean.

1 files changed, 80 insertions, 0 deletions

diff --git a/packages/tool-youtube-transcript/src/client.ts b/packages/tool-youtube-transcript/src/client.ts
new file mode 100644
index 0000000..a088d7d
--- /dev/null
+++ b/packages/tool-youtube-transcript/src/client.ts
@@ -0,0 +1,80 @@
+/**
+ * TranscriptClient — the injected outermost edge for the youtube_transcript
+ * tool.
+ *
+ * All effects (fetch, clock-via-abort-timeout) are injected so the pure
+ * decision logic remains testable without real I/O. The factory builds a
+ * single `getTranscript` method over a self-hosted transcriber instance (no
+ * API key). Mirrors the tool-web-search FirecrawlClient's request structure:
+ * per-request timeout combined with the caller's cancellation signal via
+ * `AbortSignal.any`.
+ */
+
+import type { TranscriptResponse } from "./format.js";
+
+export type FetchLike = typeof globalThis.fetch;
+
+export const DEFAULT_BASE_URL = "http://100.102.55.49:41090";
+export const DEFAULT_TIMEOUT_MS = 30_000;
+
+export interface TranscriptClient {
+	readonly getTranscript: (url: string, signal: AbortSignal) => Promise<TranscriptResponse>;
+}
+
+export interface TranscriptClientDeps {
+	readonly baseUrl: string;
+	readonly fetchFn: FetchLike;
+	readonly timeoutMs?: number;
+}
+
+/**
+ * Create a TranscriptClient. `getTranscript` builds the request URL
+ * (`${baseUrl}/api/transcript?url=${encodeURIComponent(url)}`), calls the
+ * injected `fetchFn`, and handles HTTP + JSON errors. The per-request timeout
+ * is combined with the caller's cancellation signal via `AbortSignal.any`.
+ */
+export function createTranscriptClient(deps: TranscriptClientDeps): TranscriptClient {
+	const baseUrl = deps.baseUrl;
+	const fetchFn = deps.fetchFn;
+	const timeoutMs = deps.timeoutMs ?? DEFAULT_TIMEOUT_MS;
+
+	return {
+		async getTranscript(url: string, signal: AbortSignal): Promise<TranscriptResponse> {
+			const endpoint = `${baseUrl}/api/transcript?url=${encodeURIComponent(url)}`;
+			const controller = new AbortController();
+			const timeout = setTimeout(() => controller.abort(), timeoutMs);
+			const combined = AbortSignal.any([signal, controller.signal]);
+			try {
+				let response: Response;
+				try {
+					response = await fetchFn(endpoint, {
+						method: "GET",
+						headers: { Accept: "application/json" },
+						signal: combined,
+					});
+				} catch (err) {
+					if (signal.aborted) {
+						throw new Error("Request aborted.");
+					}
+					if (controller.signal.aborted) {
+						throw new Error(`Transcriber request timed out after ${timeoutMs / 1000} seconds.`);
+					}
+					throw err;
+				}
+				if (!response.ok) {
+					const text = await response.text().catch(() => "");
+					throw new Error(
+						`HTTP ${response.status} ${response.statusText}${text ? `: ${text}` : ""}`,
+					);
+				}
+				try {
+					return (await response.json()) as TranscriptResponse;
+				} catch {
+					throw new Error("Failed to parse transcriber response as JSON");
+				}
+			} finally {
+				clearTimeout(timeout);
+			}
+		},
+	};
+}