Add initial .cursorrules file to define project structure and guideli…

…nes (#2027)

core/.cursorrules

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+# Instructions
+
+Yor are an expert go developer.
+Reference rules that are used in your answers.
+
+# Project Overview
+
+This is a module implementing a backend server for crypto chat application.
+
+# Technology stack
+
+- Go 1.23
+- Protocol Buffers
+- gRPC/gRPCWeb provided by connectrpc.com library
+- HTTP/2 provided by x/net/http2 and net/http
+- Ethereum client provided by go-ethereum library
+- Postgres database provided by pgx/v5 library
+- Logging provided by go's slog library
+- require and assert test helpers provided by github.com/stretchr/testify
+- OpenMetrics provided by prometheus
+- Tracing provided by opentelemetry
+- Command line parsing provided by cobra
+- Flags parsing provided by viper
+- Rust is used for the mls implementation
+
+# Directory structure
+
+    |-cmd  # command line commands
+    |-config  # shared config struct
+    |-contracts  # abigen generated contract bindings
+    | |-base  # bindings for base chain contracts
+    | |-river  # bindings for river chain contracts
+    | |-types  # extra helpers for working with contract generatedtypes
+    |-docker  # files for docker container
+    |-env  # shared configuration fils for supproted environments
+    |-mls  # server-side mls implementation in Rust
+    | |-mls-tools
+    | | |-crates  # ignore
+    | | |-target  # build artifacts: ignore
+    |-node  # stream node implementation
+    | |-auth  # on-chain auth implementation for river entitlements
+    | |-base  # RiverError type and other shared helpers
+    | |-crypto  # Classes to interact with blockchain
+    | |-dlog  # slog extension to provide human-readable formatting
+    | |-events  # in-memory cache for streams loaded from db, stream event parsing, miniblock generation
+    | |-http_client  # http client for making requests to other nodes
+    | |-infra  # shared infra code for node
+    | |-lint_extensions  # shared lint extensions
+    | |-logs  # ignore
+    | |-mls_service  # mls service implementation
+    | | |-mls_tools  # mls tools implementation
+    | |-nodes  # code for tracking other nodes
+    | |-notifications  # notification service implementation
+    | | |-push
+    | | |-sync
+    | | |-types
+    | |-protocol  # generated protobuf bindings
+    | | |-protocolconnect  # generated gRPC bindings
+    | |-protocol_extensions  # extra protobuf helpers
+    | |-registries  # helper classes for working with node and stream on-chain registries
+    | |-rpc  # rpc server implementation
+    | | |-render  # rpc render implementation
+    | | | |-templates  # rpc render templates
+    | | |-sync  # cross-node stream sync aggregator implementation
+    | | | |-client
+    | |-rules  # rule engine implementation for adding events to streams
+    | |-run_files  # ignore
+    | |-scrub  # stream scrubber implementation
+    | |-shared  # StreamId implementation and other shared types
+    | |-storage  # stream storage in pg database implementation
+    | |-testutils  # test helpers
+    | | |-dbtestutils  # db test helper to create and clean up test db
+    | | |-mocks  # mock implementations for testing
+    | | |-testcert  # HTTPS test certs for testing
+    | | |-testfmt  # test formatting helpers
+    | |-utils  # shared utils
+    |-river_node  # river node main
+    | |-version  # version of the node
+    |-run_files  # ignore: local test environment deployment files
+    |-scripts  # support scripts
+    |-tools  # support tools
+    | |-migrate_db  # db migration tool
+    |-xchain  # xchain implementation to check on-chain entitlements
+    | |-bindings
+    | | |-erc1155
+    | | |-erc165
+    | | |-erc20
+    | | |-erc721
+    | | |-erc777
+    | | |-ierc5313
+    | |-client_simulator
+    | |-common
+    | |-contracts
+    | |-entitlement
+    | |-examples
+    | |-server
+    | |-util