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/hashline-chunk-formatter.ts

@@ -0,0 +1,52 @@
+export interface HashlineChunkFormatter {
+  push(formattedLine: string): string[]
+  flush(): string | undefined
+}
+
+interface HashlineChunkFormatterOptions {
+  maxChunkLines: number
+  maxChunkBytes: number
+}
+
+export function createHashlineChunkFormatter(options: HashlineChunkFormatterOptions): HashlineChunkFormatter {
+  const { maxChunkLines, maxChunkBytes } = options
+  let outputLines: string[] = []
+  let outputBytes = 0
+
+  const flush = (): string | undefined => {
+    if (outputLines.length === 0) return undefined
+    const chunk = outputLines.join("\n")
+    outputLines = []
+    outputBytes = 0
+    return chunk
+  }
+
+  const push = (formattedLine: string): string[] => {
+    const chunksToYield: string[] = []
+    const separatorBytes = outputLines.length === 0 ? 0 : 1
+    const lineBytes = Buffer.byteLength(formattedLine, "utf-8")
+
+    if (
+      outputLines.length > 0 &&
+      (outputLines.length >= maxChunkLines || outputBytes + separatorBytes + lineBytes > maxChunkBytes)
+    ) {
+      const flushed = flush()
+      if (flushed) chunksToYield.push(flushed)
+    }
+
+    outputLines.push(formattedLine)
+    outputBytes += (outputLines.length === 1 ? 0 : 1) + lineBytes
+
+    if (outputLines.length >= maxChunkLines || outputBytes >= maxChunkBytes) {
+      const flushed = flush()
+      if (flushed) chunksToYield.push(flushed)
+    }
+
+    return chunksToYield
+  }
+
+  return {
+    push,
+    flush,
+  }
+}