Add Go-based PokeAPI fetch tool

Replaces the Python fetch_pokeapi.py script with a Go tool that crawls
a local PokeAPI instance and writes seed JSON files. Supports caching
and special encounter definitions via JSON config.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tools/fetch-pokeapi/client.go

@@ -0,0 +1,106 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"net/http"
+	"os"
+	"path/filepath"
+	"strings"
+	"time"
+)
+
+// Client is an HTTP client for the PokeAPI with disk caching and concurrency limiting.
+type Client struct {
+	baseURL    string
+	httpClient *http.Client
+	cacheDir   string
+	sem        chan struct{} // concurrency limiter
+}
+
+// NewClient creates a new PokeAPI client.
+func NewClient(baseURL, cacheDir string, concurrency int) *Client {
+	return &Client{
+		baseURL: strings.TrimRight(baseURL, "/"),
+		httpClient: &http.Client{
+			Timeout: 2 * time.Minute,
+		},
+		cacheDir: cacheDir,
+		sem:      make(chan struct{}, concurrency),
+	}
+}
+
+// Get fetches the given endpoint, using disk cache when available.
+func (c *Client) Get(ctx context.Context, endpoint string) ([]byte, error) {
+	// Check cache first (no semaphore needed for disk reads)
+	safeName := strings.NewReplacer("/", "_", "?", "_").Replace(endpoint) + ".json"
+	cachePath := filepath.Join(c.cacheDir, safeName)
+
+	if data, err := os.ReadFile(cachePath); err == nil {
+		return data, nil
+	}
+
+	// Acquire semaphore for HTTP request
+	select {
+	case c.sem <- struct{}{}:
+		defer func() { <-c.sem }()
+	case <-ctx.Done():
+		return nil, ctx.Err()
+	}
+
+	url := c.baseURL + "/" + endpoint
+
+	var data []byte
+	maxRetries := 3
+	for attempt := range maxRetries {
+		req, err := http.NewRequestWithContext(ctx, "GET", url, nil)
+		if err != nil {
+			return nil, fmt.Errorf("creating request for %s: %w", endpoint, err)
+		}
+
+		resp, err := c.httpClient.Do(req)
+		if err != nil {
+			if attempt < maxRetries-1 {
+				backoff := time.Duration(1<<uint(attempt)) * time.Second
+				time.Sleep(backoff)
+				continue
+			}
+			return nil, fmt.Errorf("fetching %s: %w", endpoint, err)
+		}
+
+		body, readErr := io.ReadAll(resp.Body)
+		resp.Body.Close()
+
+		if resp.StatusCode != http.StatusOK {
+			if attempt < maxRetries-1 && resp.StatusCode >= 500 {
+				backoff := time.Duration(1<<uint(attempt)) * time.Second
+				time.Sleep(backoff)
+				continue
+			}
+			return nil, fmt.Errorf("fetching %s: status %d", endpoint, resp.StatusCode)
+		}
+
+		if readErr != nil {
+			return nil, fmt.Errorf("reading response for %s: %w", endpoint, readErr)
+		}
+
+		data = body
+		break
+	}
+
+	// Write to cache
+	if err := os.MkdirAll(c.cacheDir, 0o755); err != nil {
+		return nil, fmt.Errorf("creating cache dir: %w", err)
+	}
+	if err := os.WriteFile(cachePath, data, 0o644); err != nil {
+		return nil, fmt.Errorf("writing cache for %s: %w", endpoint, err)
+	}
+
+	return data, nil
+}
+
+// ClearCache removes the cache directory.
+func (c *Client) ClearCache() error {
+	return os.RemoveAll(c.cacheDir)
+}