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/hooks/create-tool-guard-hooks.ts

@@ -100,7 +100,7 @@ export function createToolGuardHooks(args: {
     : null
 
   const hashlineReadEnhancer = isHookEnabled("hashline-read-enhancer")
-    ? safeHook("hashline-read-enhancer", () => createHashlineReadEnhancerHook(ctx, { hashline_edit: { enabled: pluginConfig.hashline_edit ?? true } }))
+    ? safeHook("hashline-read-enhancer", () => createHashlineReadEnhancerHook(ctx, { hashline_edit: { enabled: pluginConfig.hashline_edit ?? false } }))
     : null
 
   const jsonErrorRecovery = isHookEnabled("json-error-recovery")