refactor(packages): extract hashline-core package

Move the hash-anchored edit core (hash computation, validation, edit operations, text normalization, chunk formatter, diff utilities, and a runtime-aware xxHash32 binding) into a new @oh-my-opencode/hashline-core workspace package.

The src/tools/hashline-edit/ surface becomes a set of thin re-export shims, so existing import paths in the plugin keep working while the pure logic lives behind a stable package boundary that has no opencode runtime dependencies.

Tests: bun test packages/hashline-core src/tools/hashline-edit

packages/hashline-core/src/diff-utils.ts

@@ -0,0 +1,53 @@
+import { createTwoFilesPatch } from "diff"
+import { computeLineHash } from "./hash-computation"
+
+export function toHashlineContent(content: string): string {
+	if (!content) return content
+	const lines = content.split("\n")
+	const lastLine = lines[lines.length - 1]
+	const hasTrailingNewline = lastLine === ""
+	const contentLines = hasTrailingNewline ? lines.slice(0, -1) : lines
+	const hashlined = contentLines.map((line, i) => {
+		const lineNum = i + 1
+		const hash = computeLineHash(lineNum, line)
+		return `${lineNum}#${hash}|${line}`
+	})
+	return hasTrailingNewline ? hashlined.join("\n") + "\n" : hashlined.join("\n")
+}
+
+export function generateUnifiedDiff(oldContent: string, newContent: string, filePath: string): string {
+	return createTwoFilesPatch(filePath, filePath, oldContent, newContent, undefined, undefined, { context: 3 })
+}
+
+export function countLineDiffs(oldContent: string, newContent: string): { additions: number; deletions: number } {
+	const oldLines = oldContent.split("\n")
+	const newLines = newContent.split("\n")
+
+	const oldSet = new Map<string, number>()
+	for (const line of oldLines) {
+		oldSet.set(line, (oldSet.get(line) ?? 0) + 1)
+	}
+
+	const newSet = new Map<string, number>()
+	for (const line of newLines) {
+		newSet.set(line, (newSet.get(line) ?? 0) + 1)
+	}
+
+	let deletions = 0
+	for (const [line, count] of oldSet) {
+		const newCount = newSet.get(line) ?? 0
+		if (count > newCount) {
+			deletions += count - newCount
+		}
+	}
+
+	let additions = 0
+	for (const [line, count] of newSet) {
+		const oldCount = oldSet.get(line) ?? 0
+		if (count > oldCount) {
+			additions += count - oldCount
+		}
+	}
+
+	return { additions, deletions }
+}