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/EXISTING_REPO_CHECKLIST.md

@@ -1,6 +1,6 @@
 ---
 title: Existing Repo Checklist
-last_modified: 2026-10-03
+last_modified: 2026-10-04
 ---
 
 Use this checklist when beginning work in a repo that may not yet conform to our
@@ -59,26 +59,28 @@ with your task.
       here run anywhere other than the repo root, the anchored entry misses
       `services/api/.claude/`: add anchored entries for those directories.
 - [ ] If the repo embeds a version in a binary: `.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 `docker build .` with
-      no build arguments must succeed; a Dockerfile that refuses an empty build
-      argument drops that refusal and keeps the argument. `script/docker` and
-      `script/cibuild` pass the version they compute on the host; it takes
-      precedence. A tag-derived version additionally needs `fetch-depth: 0` on
-      the CI checkout step, which clones shallow and fetches no tags by default.
+      the build context. It keeps out `.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.
+      `script/docker` and `script/cibuild` pass the version they compute on the
+      host; it takes precedence. A tag-derived version additionally needs
+      `fetch-depth: 0` on the CI checkout step, which clones shallow and fetches
+      no tags by default.
 - [ ] Gitea Actions workflow in `.gitea/workflows/` runs `script/cibuild` on
       push — reference
       `https://git.eeqj.de/sneak/prompts/raw/branch/main/.gitea/workflows/check.yml`