fix: align provider mypy/test with op_engine pattern

[jc-macdonald]

Fix provider mypy type-checking failures caused by incorrect mypy configuration in the provider subpackage.

Problem

mypy-provider fails with:

Module "op_system" does not explicitly export attribute "CompiledRhs"
Module "op_system" does not explicitly export attribute "compile_spec"

Root cause: the provider's [tool.mypy] in pyproject.toml set mypy_path = "$MYPY_CONFIG_FILE_DIR/src" and explicit_package_bases = true, which caused mypy to resolve src/flepimop2/system/op_system/__init__.py as the top-level module op_system — shadowing the installed op_system package.

Fix

Align the provider build/check pattern with op_engine's working approach:

1. Remove [tool.mypy] from flepimop2-op_system/pyproject.toml — provider mypy is driven entirely from the justfile CLI.
2. Add provider-sync recipe — creates a clean .venv in the provider directory and installs all deps (including flepimop2 from git and op-system from the parent directory).
3. Update mypy-provider — uses provider-sync as a dependency, invokes mypy via .venv/bin/python -m mypy --strict --namespace-packages --explicit-package-bases src/flepimop2 so mypy resolves the namespace package tree correctly.
4. Update test-provider — uses provider-sync and .venv/bin/python -m pytest for the same isolation.

Validation

• just mypy-provider → Success: no issues found in 1 source file
• just test → 62 root tests + 3 provider tests pass
• just ci-ruff → all checks pass

Closes #51