Remove submodule gitlinks that no .gitmodules ever described

[tomchop]

Problem

Docker prod test fails on every branch including main, before running any of its own steps:

fatal: No url found for submodule path 'dev/yeti' in .gitmodules
The process '/usr/bin/git' failed with exit code 128

dev/yeti and dev/yeti-feeds-frontend are recorded in the tree as submodules (mode 160000), but .gitmodules has never existed in this repository's history — git log --all -- .gitmodules is empty. There is nothing to resolve them against.

Everything after checkout is skipped, so a red X on a yeti-docker PR carries no information about that PR. It bit #52.

Reproduction

git clone --recurse-submodules does not show it — with no .gitmodules it skips submodule init silently. The failing command is the one actions/checkout runs during its auth-cleanup step:

$ git clone --branch main <repo> && cd repo
$ git submodule foreach --recursive sh -c "true"
fatal: No url found for submodule path 'dev/yeti' in .gitmodules

$ # same command on this branch
$ git submodule foreach --recursive sh -c "true"
$ echo $?
0

The full command from the CI log — git submodule foreach --recursive sh -c "git config --local --name-only --get-regexp 'core\.sshCommand' ..." — also exits 0 on this branch.

Change

git rm --cached on both paths, and gitignore them. The working tree is untouched: dev/init.sh clones both, so they are checkouts rather than tracked content. dev/yeti-agents is ignored too — another checkout people keep there, currently showing as untracked noise.

Nothing is lost

The recorded revisions were already meaningless:

path	recorded commit
dev/yeti	84d623be (2024-06-12)	on the unmerged longerauth branch, 205 behind main
dev/yeti-feeds-frontend	8319f1c9 (2024-05-27)	on main, ~2 years stale

Relationship to #43

#43 removes these same gitlinks as part of a larger move to sibling checkouts, and adds dev/source-refs.env as a real replacement for the pin. This is the minimal subset, so CI can start being meaningful while #43 takes the time its layout decision needs. It shrinks #43's diff rather than conflicting with it.