chore: remove completed ESM migration docs and clean up unused variables

Remove ESM_MIGRATION_PLAN.md as migration is complete and documented
in commit history. Clean up unused variables and imports in rule files:

- content.js: Remove unused 'visit' import and 'afterText' variable
- structural.js: Remove unused 'text' destructuring in checkHeadingHierarchy
- technical.js: Improve URL validation security with proper hostname checks

These changes improve code quality without affecting functionality.

Author: longieirl

docs-linter/ESM_MIGRATION_PLAN.md

@@ -5,8 +5,6 @@
  * based on improvements identified in KM feedback patterns.
  */
 
-import { visit } from 'unist-util-visit';
-
 class ContentRules {
   constructor() {
     this.ruleSet = [
@@ -48,7 +46,6 @@ class ContentRules {
       patterns.content.forEach(pattern => {
         if (pattern.before && pattern.after) {
           const beforeText = pattern.before.toLowerCase();
-          const afterText = pattern.after.toLowerCase();
 
           lines.forEach((line, index) => {
             const lowerLine = line.toLowerCase();

docs-linter/src/rules/content.js

@@ -134,7 +134,7 @@ class StructuralRules {
     let previousDepth = 0;
 
     headings.forEach((heading, index) => {
-      const { depth, line, text } = heading;
+      const { depth, line } = heading;
 
       // Check for skipped heading levels
       if (depth > previousDepth + 1) {
@@ -329,9 +329,10 @@ class StructuralRules {
 
     visit(ast, 'heading', (node) => {
       if (node.children && node.children[0] && node.children[0].type === 'text') {
+        const text = node.children[0].value;
         headings.push({
           depth: node.depth,
-          text: node.children[0].value,
+          text,
           line: node.position ? node.position.start.line : null
         });
       }