feat(core): rename `meta.required_provider` to `meta.provider`, keeping backward compatibility

[refeed]

The policy metadata key meta.required_provider should be simply meta.provider. The engine reads
the key in src/tirith/core/core.py (start_policy_evaluation_from_dict, around line 370) and
echoes it into the result metadata a few lines below.

Tasks

• Accept both keys: provider preferred, required_provider still working so every existing
  policy keeps functioning unchanged (optionally a deprecation note in --verbose output).
• Keep the result metadata consistent whichever spelling the policy used.
• Tests covering both spellings, and precedence when both are present.
• Update the README, docs and example policies to the new key.