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/slack.go

@@ -0,0 +1,20 @@
+package notify
+
+import (
+	"context"
+	"fmt"
+)
+
+// slackPayload matches the Slack Incoming Webhook contract: a single "text"
+// field, interpreted as Slack mrkdwn.
+type slackPayload struct {
+	Text string `json:"text"`
+}
+
+func newSlackPayload(msg Message) slackPayload {
+	return slackPayload{Text: fmt.Sprintf("*Released %s*\n\n%s", msg.Version, msg.Notes)}
+}
+
+func sendSlack(ctx context.Context, webhookURL string, msg Message) error {
+	return postJSON(ctx, webhookURL, newSlackPayload(msg))
+}