Sharpen the public README front door

[tcballard]

Summary

• lead with the merge-gate value of AsDecided CI
• explain that actions delegate policy to the native decided CLI
• add a minimal gatekeeper workflow example
• document SHA pinning for immutable action references
• correct the historical core repository label

Why

The existing README opened with repository topology and migration history.
This gives a first-time visitor an immediate explanation of what the actions do
and how to adopt one, while retaining the architectural detail below.

Validation

• git diff --check
• README front-door audit passed
• confirmed the public v1 action tag exists
• documentation-only change

[tcballard]

Closing as superseded. Current main already reflects the six-capability CI suite, including deterministic Sentry code enforcement, the Rust-only Core cutover, and the public distribution facades. This branch's RAC-era opening copy is no longer a safe base for README work.