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, 32 insertions, 0 deletions

diff --git a/packages/tool-youtube-transcript/src/extension.ts b/packages/tool-youtube-transcript/src/extension.ts
new file mode 100644
index 0000000..0669fa5
--- /dev/null
+++ b/packages/tool-youtube-transcript/src/extension.ts
@@ -0,0 +1,32 @@
+/**
+ * tool-youtube-transcript extension — registers the `youtube_transcript` tool
+ * backed by a self-hosted transcriber service on activation.
+ *
+ * The base URL comes from `YOUTUBE_TRANSCRIBER_URL` (env) with a Tailscale
+ * default. Effects (`globalThis.fetch`) come from the ambient edge here, in
+ * the shell — never in the pure core. Logging is left to the host via
+ * `host.logger`/`ctx.log` (no `console.*`, no hand-rolled logger).
+ */
+
+import type { Extension, HostAPI, Manifest } from "@dispatch/kernel";
+import { createTranscriptClient, DEFAULT_BASE_URL } from "./client.js";
+import { createYoutubeTranscriptTool } from "./tool.js";
+
+export const manifest: Manifest = {
+	id: "tool-youtube-transcript",
+	name: "YouTube Transcript Tool",
+	version: "0.0.0",
+	apiVersion: "^0.1.0",
+	trust: "bundled",
+	activation: "eager",
+	capabilities: { network: true },
+	contributes: { tools: ["youtube_transcript"] },
+};
+
+export function activate(host: HostAPI): void {
+	const baseUrl = process.env.YOUTUBE_TRANSCRIBER_URL ?? DEFAULT_BASE_URL;
+	const client = createTranscriptClient({ baseUrl, fetchFn: globalThis.fetch });
+	host.defineTool(createYoutubeTranscriptTool({ client }));
+}
+
+export const extension: Extension = { manifest, activate };