docs(captain): observable questions are not the person's to answer

Captain-side reading of the rule shipped in plan-tune/SKILL.md.tmpl and
docs/proactive.md (#3493, pstack item 4): a question a command, render or
measurement can settle is settled by the thread; a Captain that receives one
sends it back with the check that answers it. gstack holds the how; the Drive
copy of captain.md syncs from main.

Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

capy/skill/gstack/captain.md

@@ -26,6 +26,11 @@ and the Captain's drive) is the `/captain` skill and `docs/captain.md` in the gs
   - a change to a threshold, budget, trial count or test kind;
   - closing someone's contribution without landing it;
   - anything destructive or irreversible.
+- **Observable questions are not the person's to answer.** A question a command, a render or a measurement
+  can settle ("is the new path faster", "does the endpoint take this payload", "does the test still fail")
+  is settled by the thread: it runs the thing and reports the result. Only a product or preference call
+  goes up, with the observation already made. A Captain that receives an observable question sends it
+  back with the check that answers it instead of relaying it.
 - **What a gstack workflow does in Capy.** Launched with `GSTACK_SESSION_KIND=unattended`, a workflow takes
   each recommended option, writes everything else as a pending gate item, and stops. Your job is to carry
   those pending items to the Captain or person in the shape below, then feed the answer back.