feat(cli): unify uwf CAS store with global json-cas store

This resolves issue #573 by moving uwf's CAS directory from
~/.uncaged/workflow/cas/ to the shared ~/.uncaged/json-cas/ location.

Changes:
- Added getGlobalCasDir() function that respects UNCAGED_CAS_DIR env var
- Updated createUwfStore() to use the global CAS directory
- Updated all tests to set UNCAGED_CAS_DIR for test isolation
- Added comprehensive test suite for global CAS functionality
- Updated documentation (CLAUDE.md, README.md) to reflect new architecture

Benefits:
- Cross-tool visibility: json-cas CLI can now read uwf-created nodes
- Schema sharing: both tools access the same schema registry
- Future-proofing: enables json-cas render/verbose features for uwf data

Workflow metadata (threads.yaml, registry.yaml, history.jsonl) remains in
~/.uncaged/workflow/ as intended.

All tests pass. No breaking changes to existing functionality.

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

CLAUDE.md

@@ -270,7 +270,7 @@ node scripts/publish-all.mjs --dry-run    # preview without publishing
 examples/solve-issue.yaml       — write a workflow YAML definition
   │  uwf workflow put
   ▼
-~/.uncaged/workflow/cas/        — Workflow stored as CAS node
+~/.uncaged/json-cas/            — Workflow stored as CAS node (unified CAS store)
 ~/.uncaged/workflow/registry.yaml — name → hash mapping updated
   │  uwf thread start <name> -p "..."
   ▼