fix: synchronized snapshot-tag checks and fixed ANSI width emoji handling

- Introduced a shared missingSnapshotTagMessage helper and reused it from both hashline patcher and preview validation.
- Required snapshot tags in preview hashline sections for all edits, including head/tail inserts, so preview failures now match apply-time rejections.
- Updated TUI width calculation to strip ANSI escapes before grapheme splitting, preserving styled ZWJ emoji width behavior.

packages/coding-agent/test/edit-diff.test.ts

@@ -2,7 +2,7 @@ import { afterEach, beforeEach, describe, expect, test } from "bun:test";
 import * as fs from "node:fs/promises";
 import * as os from "node:os";
 import * as path from "node:path";
-import { formatHashlineHeader, InMemorySnapshotStore } from "@oh-my-pi/hashline";
+import { formatHashlineHeader, InMemorySnapshotStore, missingSnapshotTagMessage } from "@oh-my-pi/hashline";
 import {
 	adjustIndentation,
 	computeEditDiff,
@@ -251,18 +251,40 @@ describe("computeHashlineDiff", () => {
 
 	test("accepts hashline input edits", async () => {
 		const sourcePath = path.join(tempDir, "source.txt");
-		await Bun.write(sourcePath, "first\n");
+		const text = "first\n";
+		await Bun.write(sourcePath, text);
 
+		const snapshotStore = new InMemorySnapshotStore();
+		const tag = snapshotStore.record(sourcePath, text);
 		const result = await computeHashlineDiff(
-			{ input: `¶${sourcePath}\ninsert tail:\n+second` },
+			{ input: `${formatHashlineHeader(sourcePath, tag)}\ninsert tail:\n+second` },
 			tempDir,
-			new InMemorySnapshotStore(),
+			snapshotStore,
 		);
 		expect("diff" in result).toBe(true);
 		if ("diff" in result) {
 			expect(result.diff).toContain("second");
 		}
 	});
+
+	test("rejects a tagless head/tail insert in the preview path, matching apply", async () => {
+		const relativePath = "source.txt";
+		await Bun.write(path.join(tempDir, relativePath), "first\n");
+
+		// A tagless `insert tail:` carries no anchored edit, yet the apply path
+		// (Patcher.prepare) rejects it for the missing mandatory tag. The
+		// preview/diff path MUST emit the SAME rejection so a successful preview
+		// never precedes a failing apply.
+		const result = await computeHashlineDiff(
+			{ input: `¶${relativePath}\ninsert tail:\n+second` },
+			tempDir,
+			new InMemorySnapshotStore(),
+		);
+		expect("error" in result).toBe(true);
+		if ("error" in result) {
+			expect(result.error).toBe(missingSnapshotTagMessage(relativePath));
+		}
+	});
 	test("returns a handled error when the source path is a local URL", async () => {
 		const result = await computeHashlineDiff(
 			{ input: "¶local://PLAN.md\ninsert tail:\n+x" },