Fix binary data in error output when downloading GPG keys

[Copilot]

Binary GPG keys (non-ASCII-armored) caused error messages to include raw binary data in terminal output, making logs unreadable.

Changes

• armor/main.go: Remove raw line content from error messages to prevent binary data leakage
• main.go: Detect GPG key format (ASCII-armored vs binary) and handle accordingly
  • Check for GPG packet tag magic numbers (0x95, 0x99) when armor parsing fails
  • Use binary keys directly without parsing
  • Add named constants for packet tags

Example

Before:

Downloading GPG key from: https://cli.github.com/packages/githubcli-archive-keyring.gpg
��^��*^A^P^@�Z�|�^K��^S��@ûV��ǿ^P-��u�o�a֭2�a��line found. Found '��-��u'

After:

Downloading GPG key from: https://cli.github.com/packages/githubcli-archive-keyring.gpg
[dry-run] Would add repository: deb [arch=amd64 signed-by=/usr/share/keyrings/...] ...

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