Task: Implement the add command

Add a compass add <type> [flags] command that creates a v2 document by calling ores.codegen's v2_doc_generate as a library, defaulting --parent-dir from the current sprint/version via Locate where the choice is unambiguous.

• compass add story|task|sprint|... creates the artefact via v2_doc_generate.main([...]) (in-process import, not a subprocess, not a copied generator).
• add story with no --parent-dir lands in the current sprint; add sprint in the current version; add task requires explicit --parent-dir.
• add --help documents the usage; pystache is declared in compass's requirements.txt (only add needs it).
• where / search / list / show continue to work without pystache.

1. Confirm v2_doc_generate is importable (main(argv) already exists).
2. Add a shared current_version_sprint() helper (used by where and add); add _default_parent_dir and cmd_add; short-circuit add in main() and register it for --help.
3. Lazy-import v2_doc_generate inside cmd_add (optional pystache dep); add pystache to requirements.txt.
4. Smoke-test (add story into the current sprint; clean up).

Dogfooded: this very task file was created with compass add task.