Phase 2: add writeGlobalSettings — merge into global settings.json.

Preserves unrelated keys in ~/.pi/agent/settings.json when persisting
/skill-reinject global on|off and other skillReinject fields.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/settings.ts

@@ -1,5 +1,7 @@
 import type { ExtensionContext } from "@earendil-works/pi-coding-agent";
 import { SettingsManager, getAgentDir } from "@earendil-works/pi-coding-agent";
+import { existsSync, mkdirSync, readFileSync, writeFileSync } from "fs";
+import { dirname, join } from "path";
 import type { AutoCompactIntegration } from "./state";
 
 /** JSON key in global/project settings.json (SPEC §7.3). */
@@ -101,3 +103,35 @@ export function readSettings(ctx: ExtensionContext): SkillReinjectSettings {
 		extractSkillReinject(manager.getProjectSettings()),
 	);
 }
+
+function readSettingsFile(settingsPath: string): Record<string, unknown> {
+	if (!existsSync(settingsPath)) {
+		return {};
+	}
+	try {
+		const parsed: unknown = JSON.parse(readFileSync(settingsPath, "utf8"));
+		if (!parsed || typeof parsed !== "object" || Array.isArray(parsed)) {
+			return {};
+		}
+		return parsed as Record<string, unknown>;
+	} catch {
+		return {};
+	}
+}
+
+/** Merge skillReinject into a settings.json file without replacing unrelated keys (SPEC §7.3). */
+export function mergeSkillReinjectIntoSettingsFile(
+	settingsPath: string,
+	partial: PartialSkillReinjectSettings,
+): void {
+	const current = readSettingsFile(settingsPath);
+	const existing = parseSkillReinjectPartial(current[SKILL_REINJECT_SETTINGS_KEY]);
+	current[SKILL_REINJECT_SETTINGS_KEY] = { ...existing, ...partial };
+	mkdirSync(dirname(settingsPath), { recursive: true });
+	writeFileSync(settingsPath, `${JSON.stringify(current, null, 2)}\n`, "utf8");
+}
+
+/** Persist partial skillReinject settings to global ~/.pi/agent/settings.json (SPEC §7.3). */
+export function writeGlobalSettings(partial: PartialSkillReinjectSettings): void {
+	mergeSkillReinjectIntoSettingsFile(join(getAgentDir(), "settings.json"), partial);
+}