docs: add AGENTS.md covering the conventions CI enforces

[fbwoolf]

Closes #80.

The deny list, module-level item ordering, the nightly-only rustfmt settings and the contracts-before-crates build order are only discoverable today by failing CI. This states them once at the root, linking to the README for environment setup and to the spec for the engineering standards rather than restating either.

Three calls worth reviewing:

• Added short sections on secret handling, comments and commit conventions, since those are what an agent gets wrong precisely because no lint catches them.
• One file at the root for now. contracts/ is the obvious place for a nested one if the Solidity side outgrows its section.
• The commit and branch conventions are inferred from the log rather than written down anywhere, so push back if they are not what the team wants recorded.