Split CI into parallel jobs

[dannywillems]

Summary

• Split the single build job into 4 independent jobs:
  • lint: trailing whitespace + prettier formatting checks
  • shellcheck: shellcheck on shell example scripts
  • python: Python syntax check with leakix client installed
  • build: Hugo build (runs after lint, shellcheck, python pass)
• Lint, shellcheck, and python jobs run in parallel for faster feedback
• Move version env vars (Hugo, Python, leakix client) to workflow level
• Deploy job unchanged (still gated on build + master branch)

Test plan

• All 4 CI jobs pass
• Deploy still works on master push