TextForSpeech v0.15.1

What changed

tightened the repository guidance in AGENTS.md so it now reflects TextForSpeech as a package-specific normalization library instead of carrying the broader bootstrap template
added CONTRIBUTING.md as a repo-facing contributor guide covering the SwiftPM workflow, architecture boundaries, and doc-sync expectations
updated README.md to point contributors at the new contributing guide from the main package documentation surface

no code changes are required for package consumers
maintainers should use CONTRIBUTING.md for the repo workflow baseline and keep it aligned with AGENTS.md and docs/maintainers/ when public API or architecture changes land