Add Telegram Integration plugin

[spinnakergit]

Summary

• Adds telegram plugin to the index
• Plugin repo: https://github.com/spinnakergit/a0-telegram
• 6 tools (read, send, members, summarize, manage, chat), 3 skills, real-time chat bridge
• Dual-mode security architecture (restricted/elevated) with HMAC auth, brute-force lockout, input sanitization
• 58/58 automated tests, 76/76 human verification tests, security assessment complete

Test plan

• CI validates index.yaml structure and fields
• CI validates remote plugin.yaml name: telegram matches folder
• CI validates title/description/tag length constraints

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