chore: add CLAUDE.md with development guide (#16510)

CLAUDE.md

@@ -0,0 +1,35 @@
+# Loki Development Guide
+
+## Build & Test Commands
+```bash
+make all                      # build all binaries
+make loki                     # build loki only
+make logcli                   # build logcli only
+make test                     # run all unit tests
+make test-integration         # run integration tests
+go test ./...                 # run all tests with Go directly
+go test -v ./pkg/logql/...    # run tests in specific package
+go test -run TestName ./pkg/path  # run a specific test
+make lint                     # run all linters
+make format                   # format code (gofmt and goimports)
+```
+
+## Code Style Guidelines
+- Follow standard Go formatting (gofmt/goimports)
+- Import order: standard lib, external packages, then Loki packages
+- Error handling: Always check errors with `if err != nil { return ... }`
+- Use structured logging with leveled logging (go-kit/log)
+- Use CamelCase for exported identifiers, camelCase for non-exported 
+- Document all exported functions, types, and variables
+- Use table-driven tests when appropriate
+- Follow Conventional Commits format: `<change type>: Your change`
+- For frontend: use TypeScript, functional components, component composition
+- Frontend naming: lowercase with dashes for directories (components/auth-wizard)
+
+## Documentation Standards
+- Follow the Grafana [Writers' Toolkit](https://grafana.com/docs/writers-toolkit/) Style Guide
+- Use CommonMark flavor of markdown for documentation
+- Create LIDs (Loki Improvement Documents) for large functionality changes
+- Document upgrading steps in `docs/sources/setup/upgrade/_index.md`
+- Preview docs locally with `make docs` from the `/docs` directory
+- Include examples and clear descriptions for public APIs