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>

tests/testjet.jet

@@ -1,5 +1,5 @@
 #!../jet-2.0
-<jet cgi="true" name1="localname" filterblanklines="true" trimlines="true">
+<jet cgi="yes" name1="localname" filterblanklines="true" trimlines="true">
  <header name="Content-Type" value="text/html" />
  <comment>This is a comment and should not show up in the output.</comment>
  <html>