docs(policy): pin retirement version for deprecated [engine] block

[gmoncor]

What: Pin a concrete target version (and tracking issue) for retiring the [engine] compatibility alias, in both the code comment and the runtime DeprecationWarning message.

Why: The alias says it is a "one-release" compatibility shim but names no version or issue, so nothing owns its removal — it can (and likely will) outlive its intended lifetime, matching a known anti-pattern already called out in this project's own review conventions.

How:

• Replace "a future release" / "one-release compatibility alias" with a concrete version number (0.12.0) in the code comment and the warnings.warn message.
• Reference tracking issue Remove deprecated [engine] compatibility block in policy.py #810 for the removal, following the #149/#156/#157 convention already used elsewhere in this same file.

Testing: uv run pytest -q tests/test_policy.py — 352 passed, 2 skipped. Confirms the deprecation-warning path still matches the updated message text.

Changelog: Changed: pinned a target version for the deprecated [engine] settings block.

Related to #810

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