fix: harden fix-broken-links prompt url handling (#2083)

Sanitize URL input before embedding it in Copilot handoff prompts in both bash and PowerShell hook scripts to prevent command/prompt injection from untrusted link text.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: aaronpowell

hooks/fix-broken-links/link-fix.ps1

@@ -158,7 +158,8 @@ function Get-AgentAlts {
   param([string]$Url,[int]$Max)
   if (-not (Get-Command copilot -ErrorAction SilentlyContinue)) { return @() }
   $snappy = $AGENT_TIMEOUT - 5
-  $prompt = "In under $snappy seconds, find up to $Max working alternative URLs for the broken link $Url. Hierarchically consider 1. Path and/or page spelling; 2. web.archive.org/wayback; 3. Redirects using redirect destination; 4. The context of the link's text; in order to resolve. Output only the URLs. One per line, and no: prose, numbering, markdown, backticks, special characters, post formatting."
+  $promptUrl = Get-PromptSafeUrl $Url
+  $prompt = "In under $snappy seconds, find up to $Max working alternative URLs for the broken link $promptUrl. Hierarchically consider 1. Path and/or page spelling; 2. web.archive.org/wayback; 3. Redirects using redirect destination; 4. The context of the link's text; in order to resolve. Output only the URLs. One per line, and no: prose, numbering, markdown, backticks, special characters, post formatting."
   $out = ''
   try {
     # FIX_BROKEN_LINKS_AGENT marks the child run so a re-entrant hook exits early.
@@ -190,6 +191,16 @@ function Get-AgentAlts {
   return ,$result.ToArray()
 }
 
+# Prepare a URL for safe embedding inside a prompt string.
+# This is defense-in-depth for values that originate from document content.
+function Get-PromptSafeUrl {
+  param([string]$Url)
+  if ($null -eq $Url) { return '' }
+  $safe = $Url -replace '[\r\n]+', ' '
+  $safe = $safe -replace '[`$()]', ''
+  return $safe
+}
+
 # Up to MAX viable replacement URLs for a broken link, best first:
 #   1. a working scheme/www/slash variation (verified live 200)
 #   2. alternatives proposed by the Copilot CLI agent (see Get-AgentAlts)

hooks/fix-broken-links/link-fix.sh

@@ -171,9 +171,10 @@ find_variation() {
 # from anywhere in the output, trim trailing punctuation, drop the broken URL
 # itself, and de-duplicate (case-insensitively). Up to MAX lines, one URL each.
 agent_alts() {
-  local url="$1" max="$2" prompt out
+  local url="$1" max="$2" prompt out prompt_url
   command -v copilot >/dev/null 2>&1 || return 0
-  prompt="In under $((AGENT_TIMEOUT - 5)) seconds, find up to ${max} working alternative URLs for the broken link ${url}. Hierarchically consider 1. Path and/or page spelling; 2. web.archive.org/wayback; 3. Redirects using redirect destination; 4. The context of the link's text; in order to resolve. Output only the URLs. One per line, and no: prose, numbering, markdown, backticks, special characters, post formatting."
+  prompt_url="$(sanitize_prompt_url "$url")"
+  prompt="In under $((AGENT_TIMEOUT - 5)) seconds, find up to ${max} working alternative URLs for the broken link ${prompt_url}. Hierarchically consider 1. Path and/or page spelling; 2. web.archive.org/wayback; 3. Redirects using redirect destination; 4. The context of the link's text; in order to resolve. Output only the URLs. One per line, and no: prose, numbering, markdown, backticks, special characters, post formatting."
   # FIX_BROKEN_LINKS_AGENT marks the child run so a re-entrant hook exits early.
   out="$(FIX_BROKEN_LINKS_AGENT=1 $AGENT_RUN copilot -p "$prompt" \
           -s --no-color --model "$AGENT_MODEL" --available-tools 2>/dev/null)"
@@ -209,6 +210,17 @@ suggest_alts() {
   printf '%s\n' "${out[@]}"
 }
 
+# Prepare a URL for safe embedding inside a shell-built prompt string.
+# This is defense-in-depth for values that originate from document content.
+sanitize_prompt_url() {
+  local s="$1"
+  s="${s//$'\r'/ }"
+  s="${s//$'\n'/ }"
+  s="${s//\`/\\\`}"
+  s="${s//\$/\\\$}"
+  printf '%s' "$s"
+}
+
 # Replace a literal URL everywhere in a file (pure bash, no regex).
 replace_url() {
   local file="$1" old="$2" new="$3" content pat