chore: release v0.1.15

[e54-bot]

Summary

• prepare the workspace and package metadata for v0.1.15
• bootstrap a real release to execute and verify the integrated first-party opy-provider binary distribution path from Publish provider binaries in the release-plz release flow #176/ci: publish provider binaries in release-plz flow #178
• keep the existing release-plz workflow responsible for publishing crates, creating v0.1.15, and running the provider matrix/upload jobs after merge

Validation

• cargo fmt --all -- --check
• cargo check --locked --workspace --all-targets --all-features
• locked Cargo metadata confirms all workspace packages are 0.1.15
• git diff --check
• independent ablation confirms the pre-bump package state is identical to the published v0.1.14 package state for release-plz purposes

Refs #176
Refs #178