docs: add README describing Ante's intent and honest early status

[woksin]

Summary

Ante had no README. This adds one written strictly from what the repository and its GitHub description establish today.

What the README contains

• One-liner and problem statement taken from the repository's own GitHub description: a SaaS invitation system — create signed invitations, manage the lobby where invitees register, and coordinate via events.
• Honest status: the repository does not yet contain the Ante source code (it holds the MIT LICENSE, .ai/ development rules/skills for AI-assisted development on the Cratis stack, and .github/ workflows). No quickstart is included because there is nothing to install or run.
• Intended scope clearly labeled as intent; signature scheme, storage, and APIs are explicitly deferred until source code lands.
• Ecosystem fit grounded in verifiable repository facts: the repo is scaffolded with Cratis application-development rules (Chronicle event sourcing, Arc CQRS, vertical slices), with links to the Chronicle and Arc repositories and cratis.io.
• MIT license statement matching the LICENSE file.

Deliberately excluded

• No functionality, maturity, benchmark, or support claims — there is no source to back any.
• No invented mechanics (signing, flows, storage, APIs).

Labeled no-release. Human merges.