(sentry) Prepare the public GitHub Action facade

[tcballard]

Summary

• add the public-facing Sentry Action README and Marketplace usage contract
• pin the facade to the native As Decided v0.25.1 release
• pass diff bases as quoted arguments instead of a word-split shell fragment
• extend action and packaging tests to certify the standalone facade

Authoritative development remains in asdecided/ci; the packaged public face is intended for asdecided/sentry.

Validation

• full action-contract suite: 23 passed
• six-facade packaging: passed
• packaged Sentry facade contains its root action.yml, README, license, and native installer
• no Python or pip runtime dependency