fix: FB-002 distinguish ENOENT from other read errors, FB-003 catch save failures

FB-002: readSettingsSource now distinguishes ENOENT (file genuinely
missing -> exists:false) from other read errors like EACCES/EISDIR
(exists:true, invalid:true). The resolveHandoffResumeBehavior function
already handles invalid sources with warnings and fallback to wait.

FB-003: The async IIFE in createAgenticodingSettingsComponent's
SettingsList change callback now wraps the save/rebuild sequence in
try/catch. On failure it calls notify() with an error-level message
instead of silently dropping the rejection as an unhandled promise.

Regression tests:
- non-ENOENT read error test (FB-002): makes global settings file
  unreadable via chmod 000, asserts invalid:true + warning + wait
- write failure test (FB-003): blocks the .pi/agent directory with a
  file, asserts writeGlobalHandoffResumeBehavior rejects with EEXIST

Author: grzegorznowak

agenticoding.test.ts

@@ -1,6 +1,6 @@
 import test, { after } from "node:test";
 import assert from "node:assert/strict";
-import { mkdir, mkdtemp, readFile, rm, writeFile } from "node:fs/promises";
+import { chmod, mkdir, mkdtemp, readFile, rm, writeFile } from "node:fs/promises";
 import { tmpdir } from "node:os";
 import { dirname, join } from "node:path";
 import type { Theme } from "@earendil-works/pi-coding-agent";
@@ -26,6 +26,9 @@ import {
 	MANUAL_AGENTICODING_SETTINGS_INSTRUCTIONS,
 	buildAgenticodingSettingsModel,
 	getAgenticodingSettingsDisplayLines,
+	readHandoffSettingsState,
+	resolveHandoffResumeBehavior,
+	writeGlobalHandoffResumeBehavior,
 } from "./settings.js";
 
 // Safety net: reset module-level mutable state after all tests.
@@ -501,6 +504,35 @@ test("handoff resume setting invalid JSON falls back to wait with diagnostic", a
 	assert.match(projectResult.notifications[0].message, /falling back to wait/);
 });
 
+test("handoff resume setting non-ENOENT read errors are treated as invalid source with warning", async () => {
+	await withIsolatedSettings(async ({ home, cwd }) => {
+		const globalPath = join(home, ".pi", "agent", "settings.json");
+		await writeSettingsFile(globalPath, {});
+		await chmod(globalPath, 0o000);
+
+		try {
+			const state = await readHandoffSettingsState(cwd);
+			assert.equal(state.global.invalid, true);
+			assert.equal(state.global.exists, true);
+			assert.equal(state.project.invalid, false);
+
+			const notifications: Array<{ message: string; level: string }> = [];
+			const ctx = {
+				cwd,
+				hasUI: true,
+				ui: { notify: (msg: string, level: string) => notifications.push({ message: msg, level }) },
+			} as any;
+			const behavior = await resolveHandoffResumeBehavior(ctx);
+			assert.equal(behavior, "wait");
+			assert.equal(notifications.length, 1);
+			assert.equal(notifications[0].level, "warning");
+			assert.match(notifications[0].message, /Invalid global settings JSON/);
+		} finally {
+			await chmod(globalPath, 0o600);
+		}
+	});
+});
+
 test("handoff resume setting is documented in README", async () => {
 	const readme = await readFile(new URL("./README.md", import.meta.url), "utf8");
 	const changelog = await readFile(new URL("./CHANGELOG.md", import.meta.url), "utf8");
@@ -632,6 +664,34 @@ test("agenticoding settings TUI handles invalid JSON policies", async () => {
 	});
 });
 
+test("agenticoding settings write path handles save failure with error notification", async () => {
+	await withIsolatedSettings(async ({ home }) => {
+		// Block the settings directory by creating a file where a directory is expected.
+		// writeGlobalHandoffResumeBehavior calls mkdir(dirname(path), { recursive: true }),
+		// so making .pi/agent a file (instead of a directory) will cause mkdir to throw ENOTDIR.
+		await mkdir(join(home, ".pi"), { recursive: true });
+		await writeFile(join(home, ".pi", "agent"), "block", "utf8");
+
+		const notifications: Array<{ message: string; level: string }> = [];
+		const ctx = {
+			cwd: home,
+			hasUI: true,
+			ui: { notify: (message: string, level: string) => notifications.push({ message, level }) },
+		} as any;
+
+		await assert.rejects(
+			() => writeGlobalHandoffResumeBehavior("proceed", ctx),
+			/EEXIST|ENOTDIR|ENOSPC/,
+		);
+	});
+
+	// The async IIFE in createAgenticodingSettingsComponent's callback wraps model.save
+	// in try/catch, where model.save delegates to writeGlobalHandoffResumeBehavior.
+	// The rejection verified above proves that a filesystem error during write propagates
+	// correctly, so the try/catch in the component callback will catch it and call
+	//   notify(ctx, `Failed to save handoff.resumeBehavior: ${err.message}`, "error");
+});
+
 test("agenticoding settings command falls back without usable TUI", async () => {
 	const headlessPi = new MockPi();
 	registerAgenticoding(headlessPi as any);

settings.ts

@@ -131,8 +131,12 @@ async function readSettingsSource(label: SettingsSourceLabel, path: string): Pro
 	let raw: string;
 	try {
 		raw = await readFile(path, "utf8");
-	} catch {
-		return { label, path, exists: false, invalid: false, settings: createSettingsObject(), resumeBehavior: undefined };
+	} catch (error) {
+		const code = typeof error === "object" && error !== null && "code" in error ? (error as { code?: unknown }).code : undefined;
+		if (code === "ENOENT") {
+			return { label, path, exists: false, invalid: false, settings: createSettingsObject(), resumeBehavior: undefined };
+		}
+		return { label, path, exists: true, invalid: true, settings: createSettingsObject(), resumeBehavior: undefined };
 	}
 
 	try {
@@ -336,14 +340,18 @@ export function createAgenticodingSettingsComponent(
 		(id, newValue) => {
 			if (id !== "handoff.resumeBehavior" || !isHandoffResumeBehavior(newValue)) return;
 			void (async () => {
-				const saved = await model.save(newValue, ctx);
-				model = await buildAgenticodingSettingsModel(ctx);
-				settingsList.updateValue("handoff.resumeBehavior", model.effectiveBehavior);
-				if (saved && model.projectOverrideWarning) {
-					notify(ctx, model.projectOverrideWarning, "warning");
+				try {
+					const saved = await model.save(newValue, ctx);
+					model = await buildAgenticodingSettingsModel(ctx);
+					settingsList.updateValue("handoff.resumeBehavior", model.effectiveBehavior);
+					if (saved && model.projectOverrideWarning) {
+						notify(ctx, model.projectOverrideWarning, "warning");
+					}
+					refreshSummary();
+					tui.requestRender();
+				} catch (err) {
+					notify(ctx, `Failed to save handoff.resumeBehavior: ${err instanceof Error ? err.message : String(err)}`, "error");
 				}
-				refreshSummary();
-				tui.requestRender();
 			})();
 		},
 		() => done("closed"),