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>

packages/cli-workflow/README.md

@@ -209,4 +209,13 @@ src/
 | `~/.uncaged/workflow/.env` | API keys (referenced by `apiKeyEnv` in config) |
 | `~/.uncaged/workflow/registry.yaml` | Workflow name → CAS hash |
 | `~/.uncaged/workflow/threads.yaml` | Active thread head pointers |
-| `~/.uncaged/workflow/cas/` | Content-addressed node storage |
+| `~/.uncaged/json-cas/` | Content-addressed node storage (unified CAS store, shared with `json-cas` CLI) |
+
+### Environment Variables
+
+| Variable | Purpose | Default |
+|----------|---------|---------|
+| `UNCAGED_CAS_DIR` | Override the global CAS directory location | `~/.uncaged/json-cas` |
+| `UNCAGED_WORKFLOW_STORAGE_ROOT` | Internal override for workflow metadata storage | `~/.uncaged/workflow` |
+| `WORKFLOW_STORAGE_ROOT` | User override for workflow metadata storage | `~/.uncaged/workflow` |
+