Add AGENTS.md for AI coding agent guidance

Adopt the agents.md standard to provide AI coding agents with
project-specific context including dev environment setup, testing
commands, code conventions, and PR guidelines.

Signed-off-by: Claude <noreply@anthropic.com>

AGENTS.md

@@ -0,0 +1,98 @@
+# AGENTS.md
+
+OpenCost is a Kubernetes cost monitoring tool written in Go. It provides real-time cost allocation and cloud spend visibility.
+
+## Project Structure
+
+- `cmd/costmodel/` - Main application entry point
+- `pkg/` - Core packages (cloud providers, cost model, metrics, MCP server)
+- `core/` - Shared core library (separate Go module)
+- `modules/` - Additional source modules (prometheus-source, collector-source)
+- `ui/` - Frontend UI (separate repo: opencost/opencost-ui)
+- `kubernetes/` - Kubernetes manifests
+- `protos/` - Protocol buffer definitions
+
+## Dev Environment Setup
+
+**Prerequisites:**
+- Go 1.25+
+- Docker with `buildx`
+- `just` command runner (optional, read `justfile` for manual commands)
+- Access to a Kubernetes cluster with Prometheus
+
+**Running locally:**
+```bash
+# Port-forward Prometheus (if needed)
+kubectl port-forward svc/prometheus-server 9080:80
+
+# Run the cost model
+cd cmd/costmodel
+PROMETHEUS_SERVER_ENDPOINT="http://127.0.0.1:9080" go run main.go
+```
+
+**Building:**
+```bash
+# Build local binary
+just build-local
+
+# Or using make
+make go/bin
+```
+
+## Testing
+
+**Run all unit tests:**
+```bash
+just test
+```
+
+**Run tests for specific modules:**
+```bash
+just test-core              # Core library tests
+just test-opencost          # Main application tests
+just test-prometheus-source # Prometheus source module tests
+just test-collector-source  # Collector source module tests
+```
+
+**Run integration tests** (requires running cluster with Prometheus):
+```bash
+just test-integration
+```
+
+**Code quality:**
+```bash
+go vet ./...
+go fmt ./...
+```
+
+## Code Conventions
+
+- Follow standard Go conventions and formatting (`go fmt`)
+- Use structured logging (see existing patterns in codebase)
+- Environment variables for configuration (see `pkg/env/`)
+- Multi-cloud support: AWS, Azure, GCP, Alibaba Cloud
+- Custom pricing via CSV for on-prem clusters
+
+## PR Guidelines
+
+- Include `Signed-off-by` in commit messages (DCO required)
+- Reference related issues with `Fixes #<issue>` in commit messages
+- Run `go fmt` before submitting
+- Ensure tests pass: `just test`
+- Update `THIRD_PARTY_LICENSES.txt` when adding/updating dependencies
+
+## Key Environment Variables
+
+- `PROMETHEUS_SERVER_ENDPOINT` - Prometheus server URL (required)
+- `KUBECONFIG` - Path to kubeconfig file (optional, uses default location)
+- `CLOUD_COST_ENABLED` - Enable cloud cost ingestion
+- `CLOUD_COST_CONFIG_PATH` - Path to cloud integration config
+
+## Development with Tilt
+
+```bash
+# Start local development environment
+tilt up
+```
+
+See `Tiltfile` and `tilt-values.yaml` for configuration options.