feat(notify): release v1.8.0 — Slack/Teams/Google Chat/Telegram/webhook notifications

- Add internal/notify package: best-effort release notifications to Slack, Microsoft Teams, Google Chat, Telegram, and a generic JSON webhook; every target is independently opt-in and a failed notification never fails the release
- Add notify config section (slack_webhook_url, teams_webhook_url, google_chat_webhook_url, telegram_bot_token/telegram_chat_id, webhook_url) with matching env var fallbacks; 100% coverage plus FuzzMessagePayloads
- Wire notification sending into run() — fires after tag+push (including --no-release), skipped on --no-push/--no-commit since nothing was published yet
- Document the notify section in README, Hugo docs, and .releaser.yml template; update CLAUDE.md architecture/fuzzing tables
- Also commit the Apache License 2.0 docs-site footer link (docs/hugo.toml geekdocContentLicense), left uncommitted from a prior change

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

internal/notify/notify.go

@@ -0,0 +1,88 @@
+// Package notify sends best-effort release notifications to chat and webhook targets.
+package notify
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"time"
+)
+
+// Message is the content of a release notification, shared across all targets.
+type Message struct {
+	Version string // tag name, e.g. "v1.2.3"
+	Notes   string // release notes body
+}
+
+// Config holds the destination for every supported notification target.
+// Every field is opt-in — a target is skipped when its required fields are empty.
+type Config struct {
+	SlackWebhookURL      string
+	TeamsWebhookURL      string
+	GoogleChatWebhookURL string
+	TelegramBotToken     string
+	TelegramChatID       string
+	WebhookURL           string
+}
+
+var httpClient = &http.Client{Timeout: 15 * time.Second}
+
+// SendAll sends msg to every target configured in cfg. Each target is attempted
+// independently — a failure on one does not prevent the others from being tried.
+// Returns one error per failed target, or nil if every configured target
+// succeeded (or none were configured).
+func SendAll(ctx context.Context, cfg Config, msg Message) []error {
+	var errs []error
+
+	if cfg.SlackWebhookURL != "" {
+		if err := sendSlack(ctx, cfg.SlackWebhookURL, msg); err != nil {
+			errs = append(errs, fmt.Errorf("slack: %w", err))
+		}
+	}
+	if cfg.TeamsWebhookURL != "" {
+		if err := sendTeams(ctx, cfg.TeamsWebhookURL, msg); err != nil {
+			errs = append(errs, fmt.Errorf("teams: %w", err))
+		}
+	}
+	if cfg.GoogleChatWebhookURL != "" {
+		if err := sendGoogleChat(ctx, cfg.GoogleChatWebhookURL, msg); err != nil {
+			errs = append(errs, fmt.Errorf("google chat: %w", err))
+		}
+	}
+	if cfg.TelegramBotToken != "" && cfg.TelegramChatID != "" {
+		if err := sendTelegram(ctx, cfg.TelegramBotToken, cfg.TelegramChatID, msg); err != nil {
+			errs = append(errs, fmt.Errorf("telegram: %w", err))
+		}
+	}
+	if cfg.WebhookURL != "" {
+		if err := sendWebhook(ctx, cfg.WebhookURL, msg); err != nil {
+			errs = append(errs, fmt.Errorf("webhook: %w", err))
+		}
+	}
+
+	return errs
+}
+
+// postJSON POSTs payload as JSON to url and treats any non-2xx response as an error.
+func postJSON(ctx context.Context, url string, payload any) error {
+	body, _ := json.Marshal(payload) // payload fields are always plain strings — Marshal cannot fail here
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, url, bytes.NewReader(body))
+	if err != nil {
+		return err
+	}
+	req.Header.Set("Content-Type", "application/json")
+
+	resp, err := httpClient.Do(req)
+	if err != nil {
+		return fmt.Errorf("request failed: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+		return fmt.Errorf("returned status %d", resp.StatusCode)
+	}
+	return nil
+}