v2.0.0 - LLM-friendly command names

BREAKING CHANGES:
- ls -> list-files
- rm -> remove-file
- mkdir -> create-dir
- rmdir -> remove-dir
- rrmdir -> remove-dir-recursive
- list-files text output simplified to plain filenames
  (directories marked with / suffix, no permissions/owner/size)

Unchanged:
- get, put (same names, same behavior)
- list-files --json (still returns full metadata)
- all security boundaries (path sandboxing, injection prevention)

Why: when AI agents use lockbox as a skill, Linux-style names cause
LLMs to hallucinate flags like ls -la or rm -rf. Descriptive names
eliminate this entirely.