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 with UNCAGED_CAS_DIR support
- Updated createUwfStore() to use global CAS directory
- Added comprehensive test coverage (11 new tests)
- Updated all existing tests for environment isolation
- Updated documentation (CLAUDE.md, README.md)

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

Fixes #573

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

packages/cli-workflow/src/cli.ts

@@ -373,7 +373,12 @@ step
         process.stderr.write("invalid --quota: must be a positive integer\n");
         process.exit(1);
       }
-      const markdown = await cmdStepRead(storageRoot, stepHash as CasRef, quota, opts.prompt === true);
+      const markdown = await cmdStepRead(
+        storageRoot,
+        stepHash as CasRef,
+        quota,
+        opts.prompt === true,
+      );
       process.stdout.write(markdown.endsWith("\n") ? markdown : `${markdown}\n`);
     });
   });