Set up Cursor Cloud development environment

[danielgerlag]

Summary

Sets up the DblEntry development environment for Cursor Cloud agents and verifies it end-to-end. No application code was modified — this PR only adds AGENTS.md with durable, non-obvious setup notes.

Environment

• Installed protoc (protobuf-compiler) — required by build.rs (tonic-build compiles proto/dblentry.proto).
• Updated the Rust toolchain to latest stable (1.98) — the default 1.83 lacks edition2024, which rmcp-macros (via crates/dblentry-mcp) requires.
• Installed UI deps via npm install in ui/.
• Update script (dependency refresh on startup): cargo fetch + npm install --prefix ui.

Verification

Service	Build	Test	Lint	Run
Backend (Rust)	cargo build ✓	cargo test ✓ (123 passed, 17 Postgres ignored)	cargo clippy/cargo fmt --check fail (pre-existing on master, CI red)	cargo run -- --port 3001 ✓
Web UI (Vue/Vite)	npm run build deps installed	—	—	npm run dev ✓ (:5173)

Hello-world (backend): ran the readme lending-fund FQL script via POST /fql → LoanBookTotal: 1506.34 and a balanced trial balance matching the readme.

Hello-world (UI): created accounts + a journal through the Query page and saw the data flow to Accounts, Reports (Balance Sheet + Income Statement), and Dashboard.

Note on lint

cargo fmt --all -- --check (~700 diffs) and cargo clippy --all-targets -- -D warnings (result_large_err in generated tonic proto code) both fail on master already — the project's GitHub CI is red for the same reason. These are pre-existing and left untouched.