#3149 extension fmt output prints JavaScript string concatenation and quoted values

Problem

swamp extension fmt (seen on 20261007.183334.0-sha.888e8c39) renders its log output the way extension push did before swamp-club#3017: multi-line deno output prints as JavaScript string concatenation, and plain counts print quoted.

fmt with a file to fix:

[INF] extension·fmt: Formatted 1 TypeScript files.
[INF] extension·fmt: "/…/extensions/models/text_entropy.ts\n" +
[INF] extension·fmt: "Checked 1 file"

fmt --check with a formatting failure (project deno.json with fmt.singleQuote: true):

[ERR] extension·fmt: Quality checks failed:
[ERR] extension·fmt:   "Formatting" issues:
[ERR] extension·fmt: "from /…/text_entropy.ts:\n" +
m "npm:zod@4";\n' +
m 'npm:zod@4';\n" +

push --dry-run on the same extension prints the same diff as plain lines (6 | -import { z } from "npm:zod@4";), since #3017 fixed push's renderer only.

Expected

fmt prints multi-line output as plain lines and counts without quotes, as push does after #3017.

Repro

Any extension; swamp extension fmt <manifest> to see the quoted count and concatenation, or add {"fmt":{"singleQuote":true}} to the project deno.json and run swamp extension fmt <manifest> --check for the failure output.