feat: add reusable Telegram notification workflows

[ineedjet]

🤖 [rubykatzen/baseline]:

Add separate reusable Telegram workflows for pull request and issue activity.

• notify PR opened, ready, reopened, and merged events;
• send a scheduled digest of up to ten open non-draft PRs;
• provide optional issue-closure notifications with caller-owned filtering;
• use dedicated PR-message preparation, issue-message preparation, and generic
  Telegram delivery actions;
• pass Telegram secrets explicitly and allow callers to map different channels;
• avoid logging successful Telegram API responses;
• retire the redundant unreleased-commit notification now represented by
  Release Please PRs;
• add formatter, routing, and boundary tests plus consumer documentation.

Part of #117. Also addresses the Baseline side of #99.

🔔 🧹 ✅