feat(hashline): change hashline_edit default from true to false

Hashline edit tool and companion hooks now require explicit opt-in
via `"hashline_edit": true` in config. Previously enabled by default.

- tool-registry: hashline edit tool not registered unless opted in
- create-tool-guard-hooks: hashline-read-enhancer disabled by default
- Updated config schema comment and documentation
- Added TDD tests for default behavior

src/plugin/tool-registry.ts

@@ -113,7 +113,7 @@ export function createToolRegistry(args: {
       }
     : {}
 
-  const hashlineEnabled = pluginConfig.hashline_edit ?? true
+  const hashlineEnabled = pluginConfig.hashline_edit ?? false
   const hashlineToolsRecord: Record<string, ToolDefinition> = hashlineEnabled
     ? { edit: createHashlineEditTool() }
     : {}