chore(headless): surface v2 init failure reason in fallback warning

The catch block was swallowing the actual error, leaving operators with
"v2 init failed, falling back to v1 string-matching" and no diagnostic
to act on. Found out this session that the failure was build staleness
(packages/coding-agent dist was not rebuilt by copy-resources) — would
have been instant to diagnose if the reason had been logged.

Now: "[headless] Warning: v2 init failed (Timeout waiting for response
to init...), falling back to v1 string-matching"

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/headless.ts

@@ -1901,9 +1901,10 @@ async function runHeadlessOnce(
 	try {
 		await client.init({ clientId: "sf-headless" });
 		_v2Enabled = true;
-	} catch {
+	} catch (initErr) {
+		const reason = initErr instanceof Error ? initErr.message : String(initErr);
 		process.stderr.write(
-			"[headless] Warning: v2 init failed, falling back to v1 string-matching\n",
+			`[headless] Warning: v2 init failed (${reason}), falling back to v1 string-matching\n`,
 		);
 	}