docs(git): add submodule conventions from first-hand experience

Push submodule before parent, check for -dirty flag, use rtk git
only for parent repo operations.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitmodules

@@ -7,3 +7,6 @@
 [submodule "tests/test_helper/bats-assert"]
 	path = tests/test_helper/bats-assert
 	url = https://github.com/bats-core/bats-assert.git
+[submodule "docs/wiki"]
+	path = docs/wiki
+	url = git@git.dev.rkdr.net:Defame1297/holocron.wiki.git

core/instructions/git.md

@@ -5,3 +5,9 @@
 - Commit messages explain why, not what. Written for both humans and changelog generators.
 - Never commit secrets, credentials, or environment-specific config.
 - Use conventional commits: `feat:`, `fix:`, `docs:`, `chore:`, `refactor:`, `test:`
+
+## Submodules
+
+- When working with submodules: commit and push the submodule first, then update and push the parent repo. Pushing the parent while the submodule commit doesn't exist on the remote breaks `git submodule update` for anyone who pulls.
+- Always use `rtk git` for parent repo operations; drop into the submodule directory for submodule-specific git commands.
+- After adding a submodule, check `git status` in both the parent and the submodule — a `-dirty` flag means the submodule has uncommitted local changes that need to be committed before the parent pointer is updated.