Keep a submodule's own .git/config out of the build context (closes #88)

The canonical .dockerignore kept out .git/config and the configs under
.git/modules/, but not the config of a submodule that keeps its own .git
directory, so its credential reached the image. Both git patterns now
carry the **/ prefix.

A submodule named config or deploy/config still loses its whole git
directory, and Go's version stamping fails the build. Closing that needs
a wildcard re-include, which makes BuildKit walk every excluded directory
on every build, so the file records it as a KNOWN GAP with the remedy,
git submodule add --name. REPO_POLICIES.md and both checklists say the
same.

Model: opus-5-5

.dockerignore

@@ -18,9 +18,14 @@
 # does not need .git/config; that file can hold a credential, such as a
 # password in a remote URL or the token the CI checkout step stores there.
 # Each submodule keeps a config with the same exposure in its git directory
-# under .git/modules/, nested again for a submodule's own submodules.
-.git/config
-.git/modules/**/config
+# under .git/modules/, nested again for a submodule's own submodules, or in
+# its own .git directory when it keeps one.
+# KNOWN GAP: `**/.git/modules/**/config` also matches the git directory
+# of a submodule named `config` or `deploy/config`, so all of it stays
+# out and Go's version stamping fails the build; nothing leaks. Name
+# such a submodule without that segment: `git submodule add --name`.
+**/.git/config
+**/.git/modules/**/config
 
 # Agent scratch: one full checkout of the repo per in-flight agent.
 # Anchored because it occurs once where agents run at the repo root.