feat: add Polymarket and Kalshi prediction market extractors

Add site extractors for two prediction market platforms using their
public REST APIs (no browser/Playwright dependency needed).

Polymarket: search via Gamma API, get events/markets by slug/ID/URL.
Handles inconsistent JSON encoding (stringified arrays).

Kalshi: get markets/events by ticker/URL, search via client-side
filtering of open events (no text search API available).

Both include CLI tools and comprehensive test suites using httptest.

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

sites/polymarket/polymarket.go

@@ -0,0 +1,136 @@
+package polymarket
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+)
+
+// Market represents a single prediction market on Polymarket.
+type Market struct {
+	ID            string
+	Question      string
+	Slug          string
+	Description   string
+	Outcomes      []string
+	OutcomePrices []float64 // probabilities 0-1
+	Volume        float64
+	Volume24h     float64
+	Liquidity     float64
+	EndDate       string
+	Active        bool
+	Closed        bool
+	BestBid       float64
+	BestAsk       float64
+	LastPrice     float64
+}
+
+// URL returns the Polymarket web URL for this market.
+func (m Market) URL() string {
+	if m.Slug == "" {
+		return ""
+	}
+	return "https://polymarket.com/event/" + m.Slug
+}
+
+// Event represents a prediction market event on Polymarket, which may contain
+// one or more individual markets.
+type Event struct {
+	ID          string
+	Title       string
+	Slug        string
+	Description string
+	Markets     []Market
+	Volume      float64
+	Volume24h   float64
+	Liquidity   float64
+	Active      bool
+	Closed      bool
+}
+
+// URL returns the Polymarket web URL for this event.
+func (e Event) URL() string {
+	if e.Slug == "" {
+		return ""
+	}
+	return "https://polymarket.com/event/" + e.Slug
+}
+
+// Config holds configuration for the Polymarket extractor.
+type Config struct {
+	// BaseURL overrides the Gamma API base URL. Leave empty for the default.
+	BaseURL string
+
+	// HTTPClient overrides the HTTP client used for API requests. Leave nil for default.
+	HTTPClient *http.Client
+}
+
+// DefaultConfig is the default configuration.
+var DefaultConfig = Config{}
+
+func (c Config) validate() Config {
+	return c
+}
+
+// Search finds events matching the given query string.
+func (c Config) Search(ctx context.Context, query string) ([]Event, error) {
+	c = c.validate()
+	return c.searchAPI(ctx, query)
+}
+
+// GetEvent retrieves an event by its slug or numeric ID.
+func (c Config) GetEvent(ctx context.Context, slugOrID string) (*Event, error) {
+	c = c.validate()
+
+	// Try slug first (most common from URLs)
+	ev, err := c.getEventBySlug(ctx, slugOrID)
+	if err == nil {
+		return ev, nil
+	}
+
+	// Fall back to numeric ID
+	ev, idErr := c.getEventByID(ctx, slugOrID)
+	if idErr == nil {
+		return ev, nil
+	}
+
+	return nil, fmt.Errorf("getting event %q: slug lookup: %w", slugOrID, err)
+}
+
+// GetMarket retrieves a single market by its slug.
+func (c Config) GetMarket(ctx context.Context, slug string) (*Market, error) {
+	c = c.validate()
+	return c.getMarketBySlug(ctx, slug)
+}
+
+// GetEventFromURL extracts the event slug from a Polymarket URL and retrieves
+// the event. If the URL points to a specific market within an event, the full
+// event is still returned.
+func (c Config) GetEventFromURL(ctx context.Context, rawURL string) (*Event, error) {
+	c = c.validate()
+
+	_, slug, _, err := parsePolymarketURL(rawURL)
+	if err != nil {
+		return nil, err
+	}
+
+	return c.getEventBySlug(ctx, slug)
+}
+
+// Convenience functions using DefaultConfig.
+
+func Search(ctx context.Context, query string) ([]Event, error) {
+	return DefaultConfig.Search(ctx, query)
+}
+
+func GetEvent(ctx context.Context, slugOrID string) (*Event, error) {
+	return DefaultConfig.GetEvent(ctx, slugOrID)
+}
+
+func GetMarket(ctx context.Context, slug string) (*Market, error) {
+	return DefaultConfig.GetMarket(ctx, slug)
+}
+
+func GetEventFromURL(ctx context.Context, rawURL string) (*Event, error) {
+	return DefaultConfig.GetEventFromURL(ctx, rawURL)
+}