Update qwentts.cpp build pin

[andimarafioti]

Summary

• pin validation, PyPI, and Hugging Face wheel builds to qwentts.cpp 7df559a8ca25f66fee02970514ebe5f01dee9055
• restore automatic wheel validation for relevant pull requests
• document the repeatable upstream-pin update procedure in AGENTS.md
• refresh the README revision and validation behavior

Why

The workflows still referenced the older 9dbe7ea revision, and the
pull_request trigger had previously been removed from the validation
workflow. As a result, updating the pin through a PR could not automatically
rebuild validation wheels.

Validation

• confirmed 7df559a8ca25f66fee02970514ebe5f01dee9055 is the current upstream master SHA
• parsed all GitHub Actions workflow YAML files
• 12 passed, 1 skipped in the package test suite
• locally built and loaded the CUDA 13 aarch64 library; qt_version() reports 7df559a (2026-07-17)

The PR workflow is expected to build and attach the runner-produced validation
wheels automatically.