refactor: move workflow command handlers to workflows/_commands.py (PR-8/8)

Final PR of the __init__.py split. Moves the workflow command group out of
__init__.py into the existing workflows/ package, completing the domain-dir
layout established in PR-5 (integrations), PR-6 (presets) and PR-7
(extensions).

- New workflows/_commands.py holds the four Typer apps (workflow / catalog /
  step / step-catalog), all 25 command handlers, the six workflow-only
  helpers (_parse_input_values, _workflow_run_payload, _emit_workflow_json,
  _stdout_to_stderr_when, _validate_step_id_or_exit,
  _resolve_steps_base_dir_or_exit), and a register(app) entry point.
- workflows is already a package, so no rename is needed; intra-package
  imports change from `.workflows.x` to `.x`. The only root-helper dep
  (_require_specify_project) is reached through a call-time shim so test
  monkeypatching of specify_cli._require_specify_project keeps working.
- __init__.py drops ~1445 lines (2066 -> 621); the workflow group is
  re-attached via register(app). Dead `contextlib` import removed.
- tests/test_workflows.py: import the now-relocated _stdout_to_stderr_when
  helper from its new home (workflows._commands) instead of the package root.

No behavior change. Full suite green (3847 passed), ruff clean.

Author: darion-yaphet