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, 121 insertions, 0 deletions

diff --git a/packages/tool-youtube-transcript/src/format.test.ts b/packages/tool-youtube-transcript/src/format.test.ts
new file mode 100644
index 0000000..00b5d05
--- /dev/null
+++ b/packages/tool-youtube-transcript/src/format.test.ts
@@ -0,0 +1,121 @@
+import { describe, expect, it } from "vitest";
+import {
+	type CompletedResponse,
+	type FailedResponse,
+	formatCompleted,
+	formatFailed,
+	formatQueued,
+	formatTimestamp,
+	type QueuedResponse,
+	truncateOutput,
+} from "./format.js";
+
+describe("formatTimestamp", () => {
+	it("formats seconds as m:ss", () => {
+		expect(formatTimestamp(65)).toBe("1:05");
+		expect(formatTimestamp(723)).toBe("12:03");
+	});
+
+	it("handles zero", () => {
+		expect(formatTimestamp(0)).toBe("0:00");
+	});
+
+	it("handles minutes over 60", () => {
+		// 3700s = 61m40s; 4530s = 75m30s.
+		expect(formatTimestamp(3700)).toBe("61:40");
+		expect(formatTimestamp(4530)).toBe("75:30");
+	});
+});
+
+describe("formatCompleted", () => {
+	it("formats markdown with full text + segments", () => {
+		const data: CompletedResponse = {
+			status: "completed",
+			video_id: "vid123",
+			full_text: "Hello world.",
+			segments: [
+				{ text: "Hello world.", start: 0, duration: 2.5 },
+				{ text: "Second line.", start: 65, duration: 1.0 },
+			],
+		};
+		const out = formatCompleted("https://youtu.be/vid123", data);
+		const expected = [
+			"## Transcript for https://youtu.be/vid123",
+			"**Video ID:** vid123",
+			"",
+			"### Full text",
+			"",
+			"Hello world.",
+			"",
+			"### Timestamped segments",
+			"",
+			"[0:00] Hello world.",
+			"[1:05] Second line.",
+		].join("\n");
+		expect(out).toBe(expected);
+	});
+});
+
+describe("formatQueued", () => {
+	it("returns status + position + estimated time + pending file instruction", () => {
+		const data: QueuedResponse = {
+			status: "queued",
+			video_id: "vid456",
+			position: 3,
+			estimated_seconds: 120,
+		};
+		const now = () => 1_000_000_000_000;
+		const out = formatQueued("https://youtu.be/vid456", data, now);
+		const expectedTime = new Date(1_000_000_000_000 + 120_000).toISOString();
+		const expected =
+			`Transcript not yet available (status: queued, queue position: 3).\n` +
+			`Estimated available at: ${expectedTime} (in ~120s).\n` +
+			`You must append this video URL to .youtube_subtitles_pending in the current ` +
+			`working directory: https://youtu.be/vid456`;
+		expect(out).toBe(expected);
+	});
+
+	it("includes the processing status + pending-file instruction", () => {
+		const data: QueuedResponse = {
+			status: "processing",
+			video_id: "vid457",
+			position: 0,
+			estimated_seconds: 45.5,
+		};
+		const out = formatQueued("https://youtu.be/vid457", data, () => 0);
+		expect(out).toContain("status: processing");
+		expect(out).toContain("queue position: 0");
+		expect(out).toContain("(in ~46s)");
+		expect(out).toContain(".youtube_subtitles_pending");
+		expect(out).toContain("https://youtu.be/vid457");
+	});
+});
+
+describe("formatFailed", () => {
+	it("returns error type + details", () => {
+		const data: FailedResponse = {
+			status: "failed",
+			video_id: "vid789",
+			error: "Video unavailable",
+			error_type: "NotFoundError",
+		};
+		expect(formatFailed(data)).toBe(
+			"Transcript fetch failed. Error type: NotFoundError. Details: Video unavailable",
+		);
+	});
+});
+
+describe("truncateOutput", () => {
+	it("truncates with notice when over cap", () => {
+		const output = "a".repeat(100);
+		const result = truncateOutput(output, 50);
+		expect(result).toContain("a".repeat(50));
+		expect(result).toContain("[Output truncated: exceeded 50 characters]");
+		expect(result.length).toBeLessThan(output.length + 100);
+	});
+
+	it("returns as-is when under cap", () => {
+		expect(truncateOutput("short", 100)).toBe("short");
+		expect(truncateOutput("exact", 5)).toBe("exact");
+	});
+});