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-coding-style.md

@@ -0,0 +1,53 @@
+---
+description: "Core coding style rules: immutability, file organization, error handling, input validation"
+alwaysApply: true
+---
+
+# Coding Style
+
+## Immutability (CRITICAL)
+
+ALWAYS create new objects, NEVER mutate existing ones:
+
+```
+// Pseudocode
+WRONG:  modify(original, field, value) → changes original in-place
+CORRECT: update(original, field, value) → returns new copy with change
+```
+
+Rationale: Immutable data prevents hidden side effects, makes debugging easier, and enables safe concurrency.
+
+## File Organization
+
+MANY SMALL FILES > FEW LARGE FILES:
+- High cohesion, low coupling
+- 200-400 lines typical, 800 max
+- Extract utilities from large modules
+- Organize by feature/domain, not by type
+
+## Error Handling
+
+ALWAYS handle errors comprehensively:
+- Handle errors explicitly at every level
+- Provide user-friendly error messages in UI-facing code
+- Log detailed error context on the server side
+- Never silently swallow errors
+
+## Input Validation
+
+ALWAYS validate at system boundaries:
+- Validate all user input before processing
+- Use schema-based validation where available
+- Fail fast with clear error messages
+- Never trust external data (API responses, user input, file content)
+
+## Code Quality Checklist
+
+Before marking work complete:
+- [ ] Code is readable and well-named
+- [ ] Functions are small (<50 lines)
+- [ ] Files are focused (<800 lines)
+- [ ] No deep nesting (>4 levels)
+- [ ] Proper error handling
+- [ ] No hardcoded values (use constants or config)
+- [ ] No mutation (immutable patterns used)