Name the app's Iris line app-pin, and let main mean the PR's base

The `iris` submodule's `main` was the fork's own line -- the app's 45
commits -- and not an ancestor of upstream's main, so it sat four merged
pull requests behind PR #19's actual base. Every obvious command was
therefore wrong in a way that looked plausible: `git diff main...` showed
#10, #12, #16 and #17 as the review branch's work, which is how the parley
text migration's undo path kept being reported as #19's.

The app's line is now `app-pin`, pushed to origin, and `main` tracks
`upstream/main`. `.gitmodules` follows `app-pin`, so `submodule update
--remote` cannot drag the pin onto upstream. The recorded commit is
unchanged at 32f6ad8; only the name it is reachable by has changed.

`origin/main` on the fork is left alone: moving it is a non-fast-forward on
a shared branch.

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

.gitmodules

@@ -4,4 +4,4 @@
 [submodule "iris"]
 	path = iris
 	url = git@git.arirex.me:iris-ai/iris.git
-	branch = main
+	branch = app-pin