Keep each submodule's git config out of the build context (closes #75)

The canonical `.dockerignore` kept out `.git/config`, which can hold a credential, but not the `config` in each submodule's git directory under `.git/modules/`, nested again for a submodule's own submodules. It now also lists `.git/modules/**/config`, with one sentence in the comment above; `prompts/REPO_POLICIES.md` and both checklists say so in the same words.

The pattern stays under `.git/modules/` because `.git/**/config` would also drop a branch or tag named `config`, which `git describe` may need.

Known gap: a submodule whose name has a `config` path segment loses its whole git directory, so Go's version stamping fails the build loudly; tracked separately.

Model: opus-5-5

prompts/REPO_POLICIES.md

@@ -1,6 +1,6 @@
 ---
 title: Repository Policies
-last_modified: 2026-10-03
+last_modified: 2026-10-04
 ---
 
 This document covers repository structure, tooling, and workflow standards. Code
@@ -239,20 +239,21 @@ style conventions are in separate documents:
     - If the project requires CGO or system libraries for linting (e.g.
       `vips-dev`), install them in the lint phase with `apk add`.
     - `.dockerignore` lets `.git` into the build context. It keeps out
-      `.git/config`, which `git describe` does not need and which can hold a
-      credential: a password in a remote URL, or the token the CI checkout step
-      stores there. The stage that compiles has `git` (the Debian Go image has
-      it; an alpine one needs `apk add --no-cache git`) and takes the version
-      from the `VERSION` build argument when one is given, otherwise from
-      `git describe --tags --always`. That gives the tag on a tagged commit; on
-      a later commit, the tag, the number of commits since it and the short
-      commit (`v1.2.3-4-gabc1234`); and the short commit when no tag is
-      reachable. The stage that compiles also marks its working directory safe
-      for git (`git config --system --add safe.directory /src`): a context sent
-      as a tar stream keeps the sender's file owners, and git refuses a checkout
-      owned by another user, so the version would come out empty. `ARG VERSION`
-      has no default, and the build fails if the context carries `.git` and the
-      version still comes out empty, `dev` or `unknown`. A plain
+      `.git/config` and each submodule's `config` under `.git/modules/` at any
+      depth (`.git/modules/**/config`), which `git describe` does not need and
+      which can hold a credential: a password in a remote URL, or the token the
+      CI checkout step stores there. The stage that compiles has `git` (the
+      Debian Go image has it; an alpine one needs `apk add --no-cache git`) and
+      takes the version from the `VERSION` build argument when one is given,
+      otherwise from `git describe --tags --always`. That gives the tag on a
+      tagged commit; on a later commit, the tag, the number of commits since it
+      and the short commit (`v1.2.3-4-gabc1234`); and the short commit when no
+      tag is reachable. The stage that compiles also marks its working directory
+      safe for git (`git config --system --add safe.directory /src`): a context
+      sent as a tar stream keeps the sender's file owners, and git refuses a
+      checkout owned by another user, so the version would come out empty.
+      `ARG VERSION` has no default, and the build fails if the context carries
+      `.git` and the version still comes out empty, `dev` or `unknown`. A plain
       `docker build .` with no build arguments must succeed; a Dockerfile that
       refuses an empty build argument drops that refusal and keeps the argument.