Add markdown linting tools and scripts

- Create package.json to manage markdownlint-cli2 as a dev dependency.
- Add PowerShell script for linting Markdown files with options for fixing issues.
- Update shell script to prefer locally installed markdownlint-cli2 or fallback to npx.

scripts/lint-markdown.ps1

@@ -0,0 +1,61 @@
+# EchoHub - Markdown Lint
+#
+# Runs markdownlint-cli2 over the repo. Rules, globs, and ignores all live in
+# .markdownlint-cli2.jsonc; the linter version is pinned in package.json.
+#
+# Usage:
+#   .\scripts\lint-markdown.ps1            # lint-only, exits non-zero on violations
+#   .\scripts\lint-markdown.ps1 -Fix       # auto-fix what markdownlint can
+#
+# Requires: Node + npx on PATH. Prefers the locally installed linter
+# (`npm install` once); otherwise npx fetches the same pinned version.
+
+param(
+    [switch]$Fix
+)
+
+$ErrorActionPreference = "Stop"
+
+if (-not (Get-Command npx -ErrorAction SilentlyContinue)) {
+    Write-Host "  ERROR: npx not found on PATH. Install Node.js (https://nodejs.org) and retry." -ForegroundColor Red
+    exit 1
+}
+
+$ScriptDir = Split-Path -Parent $MyInvocation.MyCommand.Path
+$RootDir   = Split-Path -Parent $ScriptDir
+
+Push-Location $RootDir
+try {
+    # Local install is version-pinned via package.json/package-lock.json; the
+    # npx fallback pins the same version so CI and local can never drift.
+    if (Test-Path "node_modules/.bin/markdownlint-cli2") {
+        $npxArgs = @('--no-install', 'markdownlint-cli2')
+    } else {
+        $npxArgs = @('--yes', '[email protected]')
+    }
+
+    if ($Fix) {
+        Write-Host "  Auto-fix mode (--fix): markdownlint will rewrite files in place." -ForegroundColor Yellow
+        $npxArgs += '--fix'
+    }
+
+    Write-Host "  > npx $($npxArgs -join ' ')" -ForegroundColor Gray
+    & npx @npxArgs
+    $exit = $LASTEXITCODE
+
+    if ($exit -eq 0) {
+        Write-Host "  Markdown lint clean." -ForegroundColor Green
+    } elseif ($Fix) {
+        Write-Host ""
+        Write-Host "  Some issues could not be auto-fixed. Review the output above and fix manually." -ForegroundColor Yellow
+    } else {
+        Write-Host ""
+        Write-Host "  Markdown lint failed. Re-run with -Fix to auto-correct the fixable rules:" -ForegroundColor Red
+        Write-Host "    .\scripts\lint-markdown.ps1 -Fix" -ForegroundColor Yellow
+    }
+
+    exit $exit
+}
+finally {
+    Pop-Location
+}