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, 30 insertions, 0 deletions

diff --git a/packages/tool-youtube-transcript/src/validate.ts b/packages/tool-youtube-transcript/src/validate.ts
new file mode 100644
index 0000000..3a9d919
--- /dev/null
+++ b/packages/tool-youtube-transcript/src/validate.ts
@@ -0,0 +1,30 @@
+/**
+ * Pure argument validation for the youtube_transcript tool — input → output, no I/O.
+ *
+ * Validates that args is an object with a non-empty `url` string. Does NOT
+ * validate URL format — the transcriber service handles that (mirrors the
+ * opencode youtube-subtitles tool's contract). Returns the URL string on
+ * success or `{ error }` for invalid input, so the tool surfaces the message
+ * verbatim as an `isError` result.
+ */
+
+export type ValidationError = { readonly error: string };
+
+/**
+ * Validate raw tool args. Returns the URL string, or `{ error }` for invalid
+ * input — the tool surfaces the message verbatim.
+ */
+export function validateUrl(args: unknown): string | ValidationError {
+	if (args === null || args === undefined || typeof args !== "object") {
+		return { error: "Error: Arguments must be an object with a 'url' string." };
+	}
+	const obj = args as Record<string, unknown>;
+	const raw = obj.url;
+	if (typeof raw !== "string") {
+		return { error: "Error: 'url' is required and must be a string." };
+	}
+	if (raw.trim().length === 0) {
+		return { error: "Error: 'url' must not be empty." };
+	}
+	return raw;
+}