fix(coding-agent): speed up external editor launch (#6903)

* fix(coding-agent): speed up external editor launch

Fixes #6774

* refactor(coding-agent): clarify external editor handling

* refactor(coding-agent): simplify external editor result

* refactor(coding-agent): model external editor lifecycle

* fix(coding-agent): rename external editor test fake

* fix(coding-agent): secure external editor temp files

* fix(coding-agent): simplify external editor temp path

* fix(coding-agent): restore system temp editor files

packages/coding-agent/test/fixtures/fake-external-editor.mjs

@@ -0,0 +1,25 @@
+import { readdirSync, readFileSync, statSync, writeFileSync } from "node:fs";
+import { dirname } from "node:path";
+
+const capturePath = process.argv[2];
+const filePath = process.argv.at(-1);
+if (!capturePath || !filePath) {
+	process.exit(1);
+}
+
+const directory = dirname(filePath);
+writeFileSync(
+	capturePath,
+	JSON.stringify({
+		filePath,
+		content: readFileSync(filePath, "utf-8"),
+		entries: readdirSync(directory),
+		directoryMode: statSync(directory).mode & 0o777,
+	}),
+	"utf-8",
+);
+
+if (process.argv.includes("--fail")) {
+	process.exit(1);
+}
+writeFileSync(filePath, process.argv.includes("--empty") ? "" : "edited\n", "utf-8");