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/package.json

@@ -0,0 +1,21 @@
+{
+  "name": "@oh-my-opencode/hashline-core",
+  "version": "0.1.0",
+  "type": "module",
+  "private": true,
+  "description": "Pure TypeScript hashline core logic for hash-anchored edits.",
+  "exports": {
+    ".": {
+      "types": "./index.d.ts",
+      "import": "./src/index.ts"
+    }
+  },
+  "types": "./index.d.ts",
+  "scripts": {
+    "typecheck": "tsgo --noEmit -p tsconfig.json",
+    "test": "bun test src/*.test.ts"
+  },
+  "dependencies": {
+    "diff": "^9.0.0"
+  }
+}