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-patterns.md

@@ -0,0 +1,36 @@
+---
+description: "Common design patterns: skeleton projects, repository pattern, API response format"
+alwaysApply: true
+---
+
+# Common Patterns
+
+## Skeleton Projects
+
+When implementing new functionality:
+1. Search for battle-tested skeleton projects
+2. Use parallel agents to evaluate options:
+   - Security assessment
+   - Extensibility analysis
+   - Relevance scoring
+   - Implementation planning
+3. Clone best match as foundation
+4. Iterate within proven structure
+
+## Design Patterns
+
+### Repository Pattern
+
+Encapsulate data access behind a consistent interface:
+- Define standard operations: findAll, findById, create, update, delete
+- Concrete implementations handle storage details (database, API, file, etc.)
+- Business logic depends on the abstract interface, not the storage mechanism
+- Enables easy swapping of data sources and simplifies testing with mocks
+
+### API Response Format
+
+Use a consistent envelope for all API responses:
+- Include a success/status indicator
+- Include the data payload (nullable on error)
+- Include an error message field (nullable on success)
+- Include metadata for paginated responses (total, page, limit)