Migrate habitat-lab to habitat-sim agent-sensor-decoupling API

Adapt habitat-lab to the new public sensor API from habitat-sim PR #2621
(agent-sensor-decoupling branch). Key changes:

- Replace all private _sensors dict access with public sim.sensors property
- Replace _sim._sensors[name]._sensor_object with sim.get_sensor(name).sensor_object
- Replace agent._sensors with agent.sensors (live C++ subtree view)
- Simplify instance_image_nav_task sensor lifecycle to use
  sim.remove_sensor() instead of manual multi-dict cleanup
- Remove Python name-mangling (_Simulator__sensors) from debug_visualizer
  and its tests; use sim.sensors registry directly
- Update CI workflow to build habitat-sim from the PR branch

Companion to: facebookresearch/habitat-sim#2621

Author: aclegg3

habitat-lab/habitat/tasks/nav/instance_image_nav_task.py

@@ -22,7 +22,6 @@
 from habitat.tasks.nav.nav import NavigationEpisode
 from habitat.tasks.nav.object_nav_task import ObjectGoal, ObjectNavigationTask
 from habitat.utils.geometry_utils import quaternion_from_coeff
-from habitat_sim import bindings as hsim
 from habitat_sim.agent.agent import AgentState, SixDOFPose
 
 try: