refactor: centralise all preflight checks into demux class initialisation

[georgemarselis-nvi]

Currently preflight checks are scattered across individual step modules. Each
step performs its own environment, credential, and dependency checks at runtime.

Move all preflight to core.py and the demux class:

• all preflight checks run at demux.__init__() or a dedicated demux.preflight() method
• each action (step) receives a fully validated demux object and can assume all
  preconditions are met
• individual step modules expose lightweight helper functions that check only
  step-specific invariants against the already-validated demux state
• no step should ever fail due to a missing environment variable, missing binary,
  or invalid config that could have been caught at startup

Benefits:

• fail fast: bad config surfaces before any sequencing data is touched
• cleaner step modules: steps do work, not environment checks
• easier testing: preflight is one place to mock
• daemon mode: preflight runs once at daemon start, not on every run

Scope:

• demux/envsetup/check_running_environment.py
• demux/envsetup/setup_environment.py
• credential checks currently scattered in step07 and step08
• binary path checks
• SSH config policy validation