security: fix command injection, XSS, prefix validation, cycle detection

- mcp_server.py: use shlex.quote() on all shell-interpolated values to
  prevent command injection via ticket title/description/human_id
- corvid/router.py: add alphanumeric-only regex validation for environment
  prefix; add full ancestor cycle detection before setting parent_id;
  import re
- Add SECURITY_REVIEW.md

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

mcp_server.py

@@ -16,6 +16,7 @@ Configure in ~/.claude/mcp.json:
 """
 
 import os
+import shlex
 
 import httpx
 from mcp.server.fastmcp import FastMCP
@@ -68,7 +69,7 @@ def _generate_startup_script(ticket_id: int, human_id: str, title: str,
         f"Start by calling mcp__homelab-tickets__get_ticket({ticket_id}) to read the full "
         f"ticket details, then complete the work and mark the ticket as completed."
     )
-    return f'claude "{prompt}" --allowedTools "{tools_str}"'
+    return f'claude {shlex.quote(prompt)} --allowedTools {shlex.quote(tools_str)}'
 
 
 def _client() -> httpx.Client: