Remove unused stream completion stdout helper

[gold-silver-copper]

Summary

• Remove the unused public low-level streaming::stream_completion_to_stdout helper.
• Keep the high-level agent::stream_to_stdout helper, re-export, examples, and tests intact.
• Document the breaking removal in the root and rig-core changelogs.

Closes #2073.

Validation

• cargo fmt
• cargo fmt --check
• cargo check -p rig-core
• cargo check -p rig-core --all-features
• cargo check -p rig-core --all-features --examples --tests
• cargo check -p rig-core --all-targets --all-features
• cargo test -p rig-core --all-features streaming -- --nocapture
• cargo test -p rig-core --lib streaming::
• git diff --check
• exact searches confirmed stream_completion_to_stdout has no references outside changelogs and agent::stream_to_stdout remains.

Not run

• Full workspace cargo test
• Full workspace cargo clippy --all-targets --all-features
• cargo doc
• Live provider/network tests