✨ Add blueprint decode/encode commands

[sakuro]

Summary

Add factorix blueprint decode and factorix blueprint encode commands for converting between Factorio blueprint strings and JSON.

Changes

• Add Factorix::Blueprint class with decode, encode, and to_json
• Add blueprint decode command (blueprint string → pretty JSON)
• Add blueprint encode command (JSON → blueprint string, no trailing newline)
• Update shell completion scripts for all three shells

Closes #76