feat: Docking station behavior for auto-charging (Issue #489)

- Integrate saltybot_docking package into full_stack.launch.py
- Auto-trigger docking when battery drops to 20% (configurable via battery_low_pct)
- Launch docking at t=7s (after sensors, before Nav2)
- Add /saltybot/docking_state publisher (std_msgs/String) for state monitoring
- Update docking_params.yaml:
  - battery_low_pct: 15% → 20% per Issue #489
  - Add references to Issue #475 for conservative FC+hoverboard speeds
- Docking behavior includes:
  - ArUco marker or IR beacon detection for dock location
  - Nav2-based approach to pre-dock pose (~1m away)
  - Visual servoing final alignment with contact detection
  - Auto-undocking on full charge (80%) or command
  - Integration with power management for mission interruption/resumption

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

jetson/ros2_ws/src/saltybot_bringup/launch/full_stack.launch.py

@@ -194,6 +194,12 @@ def generate_launch_description():
         description="Launch rosbridge WebSocket server (port 9090)",
     )
 
+    enable_docking_arg = DeclareLaunchArgument(
+        "enable_docking",
+        default_value="true",
+        description="Launch autonomous docking behavior (auto-dock at 20% battery, Issue #489)",
+    )
+
     follow_distance_arg = DeclareLaunchArgument(
         "follow_distance",
         default_value="1.5",
@@ -418,6 +424,25 @@ def generate_launch_description():
         ],
     )
 
+    # ── t=7s  Docking (auto-dock at 20% battery, Issue #489) ──────────────────
+    docking = TimerAction(
+        period=7.0,
+        actions=[
+            GroupAction(
+                condition=IfCondition(LaunchConfiguration("enable_docking")),
+                actions=[
+                    LogInfo(msg="[full_stack] Starting autonomous docking (auto-trigger at 20% battery)"),
+                    IncludeLaunchDescription(
+                        _launch("saltybot_docking", "launch", "docking.launch.py"),
+                        launch_arguments={
+                            "battery_low_pct": "20.0",
+                        }.items(),
+                    ),
+                ],
+            ),
+        ],
+    )
+
     # ── t=14s  Nav2 (indoor only; SLAM needs ~8s to build initial map) ────────
     nav2 = TimerAction(
         period=14.0,
@@ -468,6 +493,7 @@ def generate_launch_description():
         enable_follower_arg,
         enable_bridge_arg,
         enable_rosbridge_arg,
+        enable_docking_arg,
         follow_distance_arg,
         max_linear_vel_arg,
         uwb_port_a_arg,
@@ -498,6 +524,9 @@ def generate_launch_description():
         perception,
         object_detection,
 
+        # t=7s
+        docking,
+
         # t=9s
         follower,