docs: add AGENTS.md

[realmeylisdev]

What

Adds an AGENTS.md to cdk-flutter — agent-oriented contribution guidance following the pattern already in nutshell, cashu-ts, and cdk, and being proposed for nuts.

Why

• Ecosystem alignment — the other Cashu repos already have one.
• cdk-flutter has a non-obvious build pipeline: hand-written Rust under rust/src/api/ generates Dart bindings in lib/src/rust/. Without explicit guidance an agent is likely to hand-edit the generated tree, which gets overwritten on the next just generate.
• Codifies the autonomous-vs-human-review boundary (e.g. rust/src/api/ signature changes and cdk version bumps should flag for a human; doc/formatting/test work is safe for an agent to ship directly).

Structure

• Overview / common commands — lifts the command table directly from justfile so the two stay in sync.
• Environment setup — pins flutter_rust_bridge_codegen 2.11.1 (must match the runtime pin in pubspec.yaml / rust/Cargo.toml); this is a real gotcha I hit locally (cargo install flutter_rust_bridge_codegen without --version grabs 2.12.0, which then fails codegen).
• Code Generation Boundaries — the Flutter-Rust-bridge specific section: enumerates exactly which paths are hand-written and which are generated.
• Autonomous vs. Human-Required Changes — modeled on cashubtc/nutshell/AGENTS.md.
• Architecture Overview — file tree with purpose annotations.

Scope

• Docs only. No code changes.
• No CHANGELOG.md bump.
• Does not depend on fix: update for cdk 0.16 API changes #14. Ordering between the two is maintainer's call.

Happy to adjust tone, structure, or sections to match what you'd prefer — the nutshell format was my best guess at the house style.

[realmeylisdev]

Closing for the same reason as #16 — AGENTS.md for a repo that's being replaced by cdk-dart isn't worth the review cycle. If a future Flutter wrapper on top of cdk-dart ends up wanting agent-oriented docs, happy to port this there.