Fix docs: remove non-existent Parser/set_parser references

[Copilot]

The Required Initialization section in copilot-instructions.md referenced a Parser facade class and Config.set_parser() method that don't exist in the codebase. The actual implementation uses IniParser/MsgspecParser with a private _set_parser().

Changes

• Removed "Option 2" — dropped the fabricated Parser class and Config.set_parser() usage block entirely; _set_parser is private and not a supported public API worth documenting
• Removed .env from auto-detection example — Path(".env").suffix == "" falls through to the unsupported-extension error in _detect_parser; auto-detection via set_file() does not work for .env files
• Clarified .env format entry — noted in "Supported File Formats" that EnvParser is not auto-detected by set_file()

The initialization section now only documents the actual public API:

Config.set_file(Path("config.ini"))   # Uses IniParser
Config.set_file(Path("config.json"))  # Uses MsgspecParser
Config.set_file(Path("config.yaml"))  # Uses MsgspecParser
Config.set_file(Path("config.toml"))  # Uses MsgspecParser

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[Copilot]

Pull request overview

Updates the repository’s Copilot guidance to accurately reflect confkit’s public initialization API and parser auto-detection behavior, removing references to non-existent classes/methods.

Changes:

• Removed documentation of a non-existent Parser facade and Config.set_parser() API.
• Updated initialization example to only show supported Config.set_file(...) auto-detection cases.
• Clarified that .env uses EnvParser but is not auto-detected by set_file().