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/kalshi/kalshi.go

@@ -0,0 +1,160 @@
+package kalshi
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+	"strings"
+	"time"
+)
+
+// Market represents a single prediction market on Kalshi.
+type Market struct {
+	Ticker       string
+	EventTicker  string
+	Title        string
+	Subtitle     string
+	Status       string
+	YesBid       float64 // 0-1 (probability)
+	YesAsk       float64
+	NoBid        float64
+	NoAsk        float64
+	LastPrice    float64
+	Volume       int
+	Volume24h    int
+	OpenInterest int
+	Result       string // "yes", "no", or "" if unresolved
+	Rules        string
+	CloseTime    time.Time
+}
+
+// URL returns the Kalshi web URL for this market.
+func (m Market) URL() string {
+	if m.Ticker == "" {
+		return ""
+	}
+	return "https://kalshi.com/markets/" + m.Ticker
+}
+
+// Event represents a prediction market event on Kalshi, which may contain
+// one or more individual markets.
+type Event struct {
+	EventTicker  string
+	SeriesTicker string
+	Title        string
+	Subtitle     string
+	Category     string
+	Markets      []Market
+}
+
+// URL returns the Kalshi web URL for this event.
+func (e Event) URL() string {
+	if e.EventTicker == "" {
+		return ""
+	}
+	return "https://kalshi.com/markets/" + e.EventTicker
+}
+
+// Config holds configuration for the Kalshi extractor.
+type Config struct {
+	// BaseURL overrides the 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
+
+	// MaxSearchResults limits the number of results returned by Search.
+	// Defaults to 20 if zero.
+	MaxSearchResults int
+}
+
+// DefaultConfig is the default configuration.
+var DefaultConfig = Config{}
+
+func (c Config) validate() Config {
+	if c.MaxSearchResults <= 0 {
+		c.MaxSearchResults = 20
+	}
+	return c
+}
+
+// Search finds events matching the given query string by fetching open events
+// and filtering by title. Kalshi does not provide a text search API, so this
+// performs client-side substring matching.
+func (c Config) Search(ctx context.Context, query string) ([]Event, error) {
+	c = c.validate()
+	return c.searchEvents(ctx, query, c.MaxSearchResults)
+}
+
+// GetMarket retrieves a single market by its ticker.
+func (c Config) GetMarket(ctx context.Context, ticker string) (*Market, error) {
+	c = c.validate()
+	return c.getMarketByTicker(ctx, strings.ToUpper(ticker))
+}
+
+// GetEvent retrieves an event by its ticker, including all nested markets.
+func (c Config) GetEvent(ctx context.Context, eventTicker string) (*Event, error) {
+	c = c.validate()
+	return c.getEventByTicker(ctx, strings.ToUpper(eventTicker))
+}
+
+// GetMarketFromURL extracts the market ticker from a Kalshi URL and retrieves
+// the market details.
+func (c Config) GetMarketFromURL(ctx context.Context, rawURL string) (*Market, error) {
+	c = c.validate()
+
+	ticker, err := parseKalshiURL(rawURL)
+	if err != nil {
+		return nil, err
+	}
+
+	return c.getMarketByTicker(ctx, ticker)
+}
+
+// GetEventFromURL extracts the event ticker from a Kalshi URL and retrieves
+// the event details.
+func (c Config) GetEventFromURL(ctx context.Context, rawURL string) (*Event, error) {
+	c = c.validate()
+
+	ticker, err := parseKalshiURL(rawURL)
+	if err != nil {
+		return nil, err
+	}
+
+	// The ticker from the URL could be either a market ticker or event ticker.
+	// Try as event first, then fall back to getting the market and looking up its event.
+	ev, err := c.getEventByTicker(ctx, ticker)
+	if err == nil {
+		return ev, nil
+	}
+
+	// Try as market ticker - get the market, then get its event
+	m, mErr := c.getMarketByTicker(ctx, ticker)
+	if mErr != nil {
+		return nil, fmt.Errorf("getting event from URL: %w", err)
+	}
+
+	return c.getEventByTicker(ctx, m.EventTicker)
+}
+
+// Convenience functions using DefaultConfig.
+
+func Search(ctx context.Context, query string) ([]Event, error) {
+	return DefaultConfig.Search(ctx, query)
+}
+
+func GetMarket(ctx context.Context, ticker string) (*Market, error) {
+	return DefaultConfig.GetMarket(ctx, ticker)
+}
+
+func GetEvent(ctx context.Context, eventTicker string) (*Event, error) {
+	return DefaultConfig.GetEvent(ctx, eventTicker)
+}
+
+func GetMarketFromURL(ctx context.Context, rawURL string) (*Market, error) {
+	return DefaultConfig.GetMarketFromURL(ctx, rawURL)
+}
+
+func GetEventFromURL(ctx context.Context, rawURL string) (*Event, error) {
+	return DefaultConfig.GetEventFromURL(ctx, rawURL)
+}