Unify CLI messages to English

[f3rdy]

Description

CLI user-facing messages are currently in German ("Fehler:", "Gelöscht:", "Hinzugefügt:"), while CLAUDE.md states "All code and documentation in English". For an open-source tool, English messages are more appropriate.

Scope

All click.echo() calls in src/vaultctl/cli.py with German text need to be translated to English.

Examples:

• "Fehler: Key nicht im Vault gefunden." → "Error: Key not found in vault."
• "Gelöscht:" → "Deleted:"
• "Hinzugefügt:" → "Added:"
• "Wiederhergestellt:" → "Restored:"
• "Unverändert:" → "Unchanged:"

Acceptance Criteria

• All CLI output messages are in English
• Tests updated to match new message strings
• Help texts remain English (already are)

Priority

P2

[f3rdy]

Resolved in master.