Clean up TODO.txt/testjet.jet, gitignore jetc build byproducts

TODO.txt: drop the stale "Fix the variable retriever..." bug-list entry -
long since fixed. tests/testjet.jet: cgi="true" -> cgi="yes", matching the
deliberate "yes" convention already used elsewhere (__jet.cpp checks
resolveKeyword("cgi") == "yes", not "true"). tests/dump.txt: regenerated
output from re-running the test after the above.

.gitignore: jetc/ was accumulating untracked build output (compiled .cpp,
binaries, .pic.o, .so, libCoreUtils_pic.a) alongside its real tracked
source - ignore the generated artifacts, keep the two worked-example
.jet templates explicitly un-ignored since *.cpp/no-extension patterns
would otherwise also catch demo.jet/demo2.jet's own extension-less
compiled binaries without excluding the .jet sources themselves.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitignore

@@ -5,3 +5,14 @@ docs/JetCore.aux
 docs/JetCore.dvi
 docs/JetCore.log
 docs/JetCore.toc
+
+# jetc build byproducts (compiled JET-to-C++ output, binaries, .so/.a artifacts)
+jetc/jetc
+jetc/demo
+jetc/demo2
+jetc/*.cpp
+jetc/*.pic.o
+jetc/*.so
+jetc/libCoreUtils_pic.a
+!jetc/demo.jet
+!jetc/demo2.jet