Add AGENTS.md with build/test commands and code style guidelines (#527)

Provides coding agent instructions covering:
- Build, test, and format commands
- Import conventions and vendored dependency usage
- Naming conventions (PascalCase exports, uppercase acronyms)
- Error handling patterns
- Embedding and struct tag conventions

Signed-off-by: Tamal Saha <tamal@appscode.com>

Author: tamalsaha

AGENTS.md

@@ -0,0 +1,81 @@
+# AGENTS.md - Coding Agent Instructions
+
+## Project Overview
+
+Go module (`github.com/appscode/static-assets`) providing static assets (JSON data, images, files) for AppsCode products. Uses Go embed for serving data as `fs.FS`. Module path: `appscode-cloud`. Go version: 1.21+ (CI uses 1.25).
+
+## Build & Test Commands
+
+```bash
+# Build all packages
+CGO_ENABLED=0 go build -v ./...
+
+# Run all tests
+CGO_ENABLED=0 go test -v ./...
+
+# Run a single test
+CGO_ENABLED=0 go test -v -run TestName ./path/to/package/...
+
+# Run go vet
+go vet ./...
+
+# Format code (uses vendored mod)
+hack/fmt.sh api cmd data *.go
+# Or manually:
+goimports -w <files>
+gofmt -s -w <files>
+
+# Publish static assets to S3
+make publish
+```
+
+All build/test commands require `CGO_ENABLED=0` and `GOFLAGS="-mod=vendor"`.
+
+## Code Style
+
+### Package Names
+- Lowercase, single-word: `api`, `staticassets`
+- Root package name is `staticassets` (not `static-assets`)
+
+### Imports
+- Group stdlib first, then external packages, separated by blank line
+- Use vendored dependencies (`-mod=vendor`)
+- Import the root module as: `staticassets "github.com/appscode/static-assets"`
+- Subpackages: `"github.com/appscode/static-assets/api"`
+
+```go
+import (
+	"encoding/json"
+	"os"
+
+	"github.com/appscode/static-assets/api"
+)
+```
+
+### Naming
+- Exported types/funcs: PascalCase (`Product`, `DownloadReleaseAssets`)
+- Unexported: camelCase (`exists`, `repoList`)
+- Acronyms kept uppercase: `API`, `URL`, `FS`, `ID` (e.g., `URLRef`, `RepoURL`)
+- File names: `snake_case.go` for product data files, `main.go` for commands
+
+### Struct Tags
+- Always include JSON tags on struct fields
+- Use `omitempty` for optional fields
+- Prefer explicit JSON field names: `json:"fieldName"`
+
+### Error Handling
+- Use `panic()` only for unrecoverable programmer errors (e.g., failed embed.Sub)
+- Return `error` for runtime/IO failures; handle or propagate
+- Use `log.Fatalln()` for fatal errors in CLI tools
+- Use type-switch on error types for specific handling (see GitHub API errors)
+
+### Embedding
+- Use `//go:embed` directives for static data
+- Wrap in `fs.FS` via `fs.Sub()` for clean access paths
+
+### General
+- Prefer `bytes.Buffer` + `json.NewEncoder` over `json.Marshal` for formatted JSON output
+- Use `os.WriteFile` with `0o644` permissions
+- Use `0o755` for directories
+- Keep functions small and focused
+- No comments unless explicitly requested