fix(sling): remove dead --molecule flag

The --molecule flag was defined but never wired up - the slingMolecule
variable was set by the flag parser but never read by any code path.

Users should use --on instead, which is fully implemented:
  gt sling <formula> --on <bead> <target>

The --on flag properly instantiates the formula (cook + wisp + bond)
and applies it to the target bead before slinging.

Keeping --on as the canonical way to apply formulas to beads since it's
actually wired up and working. The --molecule flag can be re-added later
if a different argument order is desired.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

docs/reference.md

@@ -242,7 +242,7 @@ Note: "Swarm" is ephemeral (workers on a convoy's issues). See [Convoys](convoy.
 # Standard workflow: convoy first, then sling
 gt convoy create "Feature X" gt-abc gt-def
 gt sling gt-abc <rig>                    # Assign to polecat
-gt sling gt-def <rig> --molecule=<proto> # With workflow template
+gt sling <proto> --on gt-def <rig>       # With workflow template
 
 # Quick sling (auto-creates convoy)
 gt sling <bead> <rig>                    # Auto-convoy for dashboard visibility