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>

bun.lock

@@ -10,6 +10,7 @@
         "@changesets/cli": "^2.31.0",
         "bun-types": "^1.3.14",
         "typescript": "^5.8.0",
+        "ulidx": "^2.4.1",
       },
     },
     "packages/cli-json-cas": {
@@ -200,6 +201,8 @@
 
     "jsonfile": ["jsonfile@4.0.0", "", { "optionalDependencies": { "graceful-fs": "^4.1.6" } }, "sha512-m6F1R3z8jjlf2imQHS2Qez5sjKWQzbuuhuJ/FKYFRZvPE3PuHcSMVZzfsLhGVOkfd20obL5SWEBew5ShlquNxg=="],
 
+    "layerr": ["layerr@3.0.0", "", {}, "sha512-tv754Ki2dXpPVApOrjTyRo4/QegVb9eVFq4mjqp4+NM5NaX7syQvN5BBNfV/ZpAHCEHV24XdUVrBAoka4jt3pA=="],
+
     "locate-path": ["locate-path@5.0.0", "", { "dependencies": { "p-locate": "^4.1.0" } }, "sha512-t7hw9pI+WvuwNJXwk5zVHpyhIqzg2qTlklJOf0mVxGSbe3Fp2VieZcduNYjaLDoy6p9uGpQEGWG87WpMKlNq8g=="],
 
     "lodash.startcase": ["lodash.startcase@4.4.0", "", {}, "sha512-+WKqsK294HMSc2jEbNgpHpd0JfIBhp7rEV4aqXWqFr6AlXov+SlcgB1Fv01y2kGe3Gc8nMW7VA0SrGuSkRfIEg=="],
@@ -282,6 +285,8 @@
 
     "typescript": ["typescript@5.9.3", "", { "bin": { "tsc": "bin/tsc", "tsserver": "bin/tsserver" } }, "sha512-jl1vZzPDinLr9eUt3J/t7V6FgNEw9QjvBPdysz9KfQDD41fQrC2Y4vKQdiaUpFT4bXlb1RHhLpp8wtm6M5TgSw=="],
 
+    "ulidx": ["ulidx@2.4.1", "", { "dependencies": { "layerr": "^3.0.0" } }, "sha512-xY7c8LPyzvhvew0Fn+Ek3wBC9STZAuDI/Y5andCKi9AX6/jvfaX45PhsDX8oxgPL0YFp0Jhr8qWMbS/p9375Xg=="],
+
     "undici-types": ["undici-types@7.24.6", "", {}, "sha512-WRNW+sJgj5OBN4/0JpHFqtqzhpbnV0GuB+OozA9gCL7a993SmU+1JBZCzLNxYsbMfIeDL+lTsphD5jN5N+n0zg=="],
 
     "universalify": ["universalify@0.1.2", "", {}, "sha512-rBJeI5CXAlmy1pV+617WB9J63U6XcazHHF2f2dbJix4XzpUF0RS3Zbj0FGIOCAva5P/d/GBOYaACQ1w+0azUkg=="],