Add comprehensive GitHub Copilot instructions for template repository

[Copilot]

This PR adds a comprehensive .github/copilot-instructions.md file that provides GitHub Copilot coding agents with detailed instructions for working effectively in this Tautulli Telegram notification template repository.

What's Added

The instructions file includes:

• Clear directive to follow the instructions first before additional search/context gathering
• Repository structure overview explaining this is a template-only repository with no build processes
• Validated commands for common tasks like template verification and content inspection
• Template structure documentation explaining the notification types and variable usage
• File location reference for frequently modified files
• Validation scenarios for ensuring template integrity after changes

Key Features

• No Build Process Documentation: Explicitly states there are no npm/pip/build commands to run since this is purely a template repository
• Template Validation Commands: Provides working commands to verify all 12 templates exist and validate their structure
• Variable Reference: Documents common Tautulli variables used in templates ({username}, {title}, {server_name}, etc.)
• HTML Formatting Guidance: Explains Telegram-specific HTML formatting requirements

Validation

All commands and procedures in the instructions have been thoroughly tested:

# Template count validation (returns 12)
find templates/ -name "*-en" -o -name "*-es" | wc -l

# Template structure verification
head -5 templates/en/playback-start-en

# Variable extraction testing
grep -o '{[^}]*}' templates/en/playback-start-en

The instructions enable any GitHub Copilot agent to work effectively with this repository without trial and error, understanding immediately that this is a template repository requiring no builds or tests.

Fixes #11.

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