fix: keep workspace ls sizes schema-safe

[chubes4]

Summary

• Always include an integer size field for local workspace-ls entries.
• Report directories and non-regular entries as size 0; report files with integer byte size.
• Add a focused smoke for the local directory listing contract.

Evidence

• SSI lab retry ssi-322-script-artifacts-retry-evidence-5 hit datamachine-code/workspace-ls output validation failures: output[entries][8][size] is not of type integer.

Testing

• php -l inc/Workspace/WorkspaceReader.php
• php -l tests/smoke-workspace-list-directory.php
• php tests/smoke-workspace-list-directory.php
• php tests/smoke-workspace-grep.php

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Drafted the schema-safe size normalization and focused smoke coverage; Chris remains responsible for review and merge.