docs: why-gates-are-code names the preflight as the second exit-42 case outside the gates (#151)

Files changed:
- docs/why-gates-are-code.md

docs/why-gates-are-code.md

@@ -83,6 +83,14 @@ that every instruction would then have to know which provider an instance runs.
 gap belongs where the capability is, and reaches the session as an exit code rather than as a
 paragraph it has to remember to apply.
 
+The preflight is the second such case, and the one closest to this page's own argument. A machine
+without Python or ripgrep is not something the tool can fix, and an install that met that gap
+with only a setup instruction to go on showed what follows: the agent worked around each missing
+piece - another environment, a hand-made configuration - and kept going. So `tools/preflight.sh`
+exits 42 with the command a human has to run, and the launcher in front of every `wikitool` call
+exits 42 until the preflight has passed. "Check first, stop, let the user act" lives in two places
+a session cannot read past, not in a step it can skip.
+
 ## A gate in code still has to be reachable
 
 Code beats prose for the reason above, but on its own it buys less than it looks like: a check