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

bun.lock

@@ -29,6 +29,7 @@
         "@oh-my-opencode/ast-grep-mcp": "workspace:*",
         "@oh-my-opencode/boulder-state": "workspace:*",
         "@oh-my-opencode/comment-checker-core": "workspace:*",
+        "@oh-my-opencode/hashline-core": "workspace:*",
         "@oh-my-opencode/model-core": "workspace:*",
         "@oh-my-opencode/rules-engine": "workspace:*",
         "@oh-my-opencode/utils": "workspace:*",
@@ -93,6 +94,13 @@
       "name": "@oh-my-opencode/comment-checker-core",
       "version": "0.1.0",
     },
+    "packages/hashline-core": {
+      "name": "@oh-my-opencode/hashline-core",
+      "version": "0.1.0",
+      "dependencies": {
+        "diff": "^9.0.0",
+      },
+    },
     "packages/model-core": {
       "name": "@oh-my-opencode/model-core",
       "version": "0.1.0",
@@ -197,6 +205,8 @@
 
     "@oh-my-opencode/comment-checker-core": ["@oh-my-opencode/comment-checker-core@workspace:packages/comment-checker-core"],
 
+    "@oh-my-opencode/hashline-core": ["@oh-my-opencode/hashline-core@workspace:packages/hashline-core"],
+
     "@oh-my-opencode/model-core": ["@oh-my-opencode/model-core@workspace:packages/model-core"],
 
     "@oh-my-opencode/rules-engine": ["@oh-my-opencode/rules-engine@workspace:packages/rules-engine"],