docs: MIGRATION.md does not document how to migrate from the old Error enum to AnchorKitError

[Haroldwonder]

Problem

src/errors.rs defines a backward-compat type alias pub type Error = AnchorKitError and a comment saying 'Prefer AnchorKitError for new code'. However, docs/MIGRATION.md does not mention this change or provide guidance on updating existing code that uses the old Error type.

Location

docs/MIGRATION.md, src/errors.rs

Fix

Add a section to MIGRATION.md explaining the Error -> AnchorKitError rename, the type alias, and any breaking changes in the error API.

[daree-dev]

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