refactor: deduplicate prompt templates with shared content system

Implemented @include() directive system to eliminate ~800 lines of duplicated content across 10 specialist prompt files. All prompt-related content now consolidated under prompts/ directory for better maintainability.

Changes:
- Added processIncludes() to prompt-manager.js for generic @include() support
- Created prompts/shared/ with 5 reusable template files
- Refactored all 10 specialist prompts to use @include() for common sections
- Moved login_instructions.txt to prompts/shared/ (deleted login_resources/)
- Updated CLAUDE.md to reflect new structure

Impact: -137 net lines, zero breaking changes, infinitely scalable for future shared content.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

prompts/vuln-auth.txt

@@ -9,16 +9,15 @@ Success criterion: A complete, code-backed analysis of every potential authentic
 </objective>
 
 <scope>
-**EXTERNAL ATTACKER SCOPE:** Only report vulnerabilities exploitable via {{WEB_URL}} from the internet. Exclude findings requiring internal network access, VPN, or direct server access.
+@include(shared/_vuln-scope.txt)
 </scope>
 
 <target>
-URL: {{WEB_URL}}
+@include(shared/_target.txt)
 </target>
 
 <rules>
-Rules to Avoid:
-{{RULES_AVOID}}
+@include(shared/_rules.txt)
 </rules>
 
 <login_instructions>