chore: prepare for public release

[kristof-siket]

Prepares the repository to be flipped public.

• Adds the Apache-2.0 license (matching prisma/composer and prisma/prisma), copyright Prisma Data, Inc.
• Rewrites the README as product documentation: what the action does, quick start with a service token, the canonical workflow pinned to v1, inputs and outputs, honest experimental status and known limitations. The internal provenance notes, fixture-repo links (private, so dead ends for public readers), and normative implementation commentary are removed; git history keeps them.
• Cleans action.yml metadata: the action is named Deploy to Prisma Cloud, descriptions are written for users, and the stale claim that destroy skips the build is corrected (both modes build).
• Updates the CI sanity check for the new action name.

No behavior changes; main.mjs and post.mjs are untouched.

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