feat: implement RFC-20 Phase 1 variable CRUD operations

Add a complete variable system for json-cas that provides mutable named
bindings to immutable CAS nodes.

Features:
- ULID-based variable identifiers (26-char Crockford Base32)
- Hierarchical scope validation (must end with /)
- Schema validation on update (prevents type mismatches)
- SQLite persistence (~/.uncaged/json-cas/variables.db)
- CLI commands: var create, get, update, delete

Implementation:
- Core types in variable.ts (Variable, VariableId)
- VariableStore class with SQLite backend
- Custom error types (VariableNotFoundError, SchemaMismatchError, etc.)
- Comprehensive unit tests (16 tests)
- CLI integration tests (12 tests)
- All outputs use JSON format

Test coverage:
- Variable creation with scope validation
- CRUD operations (create, read, update, delete)
- Schema consistency enforcement
- Error handling for all edge cases
- Full lifecycle integration tests

All tests pass (158 total), build clean, lint clean.

Fixes #21

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

package.json

@@ -9,7 +9,8 @@
     "@changesets/changelog-github": "^0.7.0",
     "@changesets/cli": "^2.31.0",
     "bun-types": "^1.3.14",
-    "typescript": "^5.8.0"
+    "typescript": "^5.8.0",
+    "ulidx": "^2.4.1"
   },
   "scripts": {
     "build": "tsc --build packages/json-cas packages/json-cas-fs",