Fix launcher autonomy environment export

[MrChikns]

Summary

• add a versioned scripts/load-plane-env.sh launcher helper
• export assignment-only plane environment files before launching Node
• auto-discover $LOOPKIT_HOME/config/autonomy.env for standalone planes
• document the launcher contract for beats and the console
• add regression coverage for assignment-only files, explicit exports, plane-home discovery, shell option restoration, and missing-file failure

Why

Sourcing a shell file containing LOOPKIT_AUTONOMY=on does not export that value to child processes. A console or beat launched afterward therefore sees the variable as unset and fails safe to OFF.

Validation

• focused launcher regression: 5/5 passed
• full @loopkit/core suite: 1,734/1,734 passed
• shell syntax check
• diff check
• staged public leak scan