revert: remove top-level move semantics

[andylokandy]

Summary

• Remove the unreleased Exn::into_error() API introduced in feat: top-level move semantics #48.
• Remove the into-error example that depended on root-only move recovery.
• Drop the corresponding unreleased changelog entry.

Closes #50.

[Copilot]

Pull request overview

This PR cleanly reverts the unreleased top-level move-recovery API from exn so the crate no longer exposes Exn::into_error() or the example/changelog entries that depended on it. It aligns the codebase with the direction discussed in #50: keeping exn focused on contextual error trees rather than root-only move semantics.

Changes:

• Remove the unreleased Exn::into_error() method from exn/src/impls.rs.
• Delete the into-error example and unregister it from examples/Cargo.toml.
• Remove the matching unreleased changelog entry.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
exn/src/impls.rs	Removes the unreleased Exn::into_error() API from the core exception type.
examples/src/into-error.rs	Deletes the example that demonstrated the removed move-recovery API.
examples/Cargo.toml	Removes the example target so Cargo no longer references the deleted file.
CHANGELOG.md	Drops the unreleased changelog note for into_error().

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