Hands: park hand tracking behind ft-handsctl

Hand tracking no longer starts with SteamVR: hands/run.sh install leaves
the units disabled and links hands/ft-handsctl into ~/.local/bin, which
turns it on and off (on | off | status | log | cutouts on|off | gestures).
With it on, hands show through the screens; pinches and grips move the
pointer only with POINTER_HANDS=1, now off by default. ft-camd's service
runs the mono cameras only: while the headset is worn, the colour module
writes just a half-size image into the top-left quarter of its buffers,
which ft-camd can't use yet.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

session/frametop.conf.example

@@ -42,7 +42,7 @@ HANDS_BRIGHT_ON=40         # hand tracking, auto: the colour frames' mean bright
 HANDS_BRIGHT_OFF=25        # ... and back to the mono cameras under this
 HANDS_COLOR_LEFT=color_video0  # hand tracking: which colour camera is passthrough_left (hands/tools/check_color.py tells)
 HANDS_COLOR_CROP=subtract  # hand tracking: how the colour calibration's crop applies, subtract | none (check_color.py tells)
-POINTER_HANDS=1            # 1 = pinches click (on the release; hold and move to nudge the pointer first) and grips press and drag, with hand tracking
+POINTER_HANDS=0            # 1 = pinches click (on the release; hold and move to nudge the pointer first) and grips press and drag, with hand tracking
 POINTER_PINCH_GAIN=0.5     # hands: the pointer moves this times the pinching hand's angle (precision)
 POINTER_PINCH_DEADZONE=1.5 # hands: degrees the pinching hand moves before the pointer does (a tap's jitter)
 POINTER_GRIP_GAIN=1        # hands: the pointer moves this times the gripping hand's angle