feat: add Cursor IDE support with pre-translated configs

Add complete .cursor/ directory with rules, agents, skills, commands,
and MCP config adapted for Cursor's format. This makes ecc-universal
a truly cross-IDE package supporting Claude Code, Cursor, and OpenCode.

- 27 rule files with YAML frontmatter (description, globs, alwaysApply)
- 13 agent files with full model IDs and readonly flags
- 30 skill directories (identical Agent Skills standard, no translation)
- 31 command files (5 multi-* stubbed for missing codeagent-wrapper)
- MCP config with Cursor env interpolation syntax
- README.md and MIGRATION.md documentation
- install.sh --target cursor flag for project-scoped installation
- package.json updated with .cursor/ in files and cursor keywords

.cursor/rules/common-security.md

@@ -0,0 +1,34 @@
+---
+description: "Mandatory security checks, secret management, and security response protocol"
+alwaysApply: true
+---
+
+# Security Guidelines
+
+## Mandatory Security Checks
+
+Before ANY commit:
+- [ ] No hardcoded secrets (API keys, passwords, tokens)
+- [ ] All user inputs validated
+- [ ] SQL injection prevention (parameterized queries)
+- [ ] XSS prevention (sanitized HTML)
+- [ ] CSRF protection enabled
+- [ ] Authentication/authorization verified
+- [ ] Rate limiting on all endpoints
+- [ ] Error messages don't leak sensitive data
+
+## Secret Management
+
+- NEVER hardcode secrets in source code
+- ALWAYS use environment variables or a secret manager
+- Validate that required secrets are present at startup
+- Rotate any secrets that may have been exposed
+
+## Security Response Protocol
+
+If security issue found:
+1. STOP immediately
+2. Use **security-reviewer** agent
+3. Fix CRITICAL issues before continuing
+4. Rotate any exposed secrets
+5. Review entire codebase for similar issues