fix(policy): warn when scm.max_parallel is silently clamped to 1

[gmoncor]

What: Emit an explicit warning when a configured scm.max_parallel > 1 is clamped to 1, instead of silently discarding the operator's intent.

Why: Phase 5 parallel fan-out is not built yet (tracked in #229), so the knob is inert — but nothing today tells an operator who sets max_parallel = 4 that their setting has no effect, which reads as the loop mysteriously running slower than configured. Related to #229, which this PR does not implement.

How:

• In loads(), warn via warnings.warn(...) when requested_parallel > 1, mirroring the existing _fold_deprecated_engine warning pattern.
• Keep the clamp behavior itself unchanged (still min(requested_parallel, 1)).
• Add a pytest.warns test asserting the warning fires for max_parallel > 1 and that the resulting policy still clamps to 1.

Testing: uv run pytest -q tests/test_policy.py.

Changelog: Changed: bmad-loop now warns when scm.max_parallel > 1 is configured, since parallel fan-out is not yet built and the value is clamped to 1.

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