feat: add --bytecode flag to debug command

[zharinov]

Summary

• Add --bytecode flag to debug command
• Dumps human-readable bytecode output from query
• Requires valid query (exits with diagnostics on error)
• Extract ensure_valid() helper for reuse

Usage:

cargo run -p plotnik-cli -- debug -q 'Test = (id) @x' --bytecode

Depends on: #192

Test plan

• cargo check -p plotnik-cli passes
• Manual test with --bytecode flag