Propagate package paths to robot drivers

[temiroff]

What changed

• build a managed-driver environment that includes the blacknode-robot package root
• add every installed extension repository under BLACKNODE_PACKAGE_PATH to child PYTHONPATH
• preserve an existing PYTHONPATH while de-duplicating resolved paths
• add a real subprocess regression test for extension-module imports

Root cause

Workflow package nodes are loaded into the deployment process dynamically. RobotDriverLauncher then starts the physical driver as a separate Python process, which inherits environment variables but not the parent process's sys.path mutations. The Feetech driver could therefore start ROS but failed on its first canonical telemetry publish when importing blacknode_robot.devices; the Runtime watchdog correctly stopped the deployment after receiving no valid telemetry.

Impact

Managed robot drivers can import the stable contracts and helper modules provided by installed extension packages. Startup remains disarmed, torque release behavior is unchanged, and the required-telemetry watchdog remains enforced.

Validation

• python -m pytest packages/blacknode-robot/tests -q with the package root on PYTHONPATH: 102 passed, 2 skipped
• focused launcher tests: 4 passed
• git diff --check