Keep in-repo agent scratch out of the build context and out of git (closes #27)

The canonical .dockerignore and .gitignore both omitted the in-repo agent
scratch directory, which holds one worktree per in-flight agent, so under
`COPY . .` an entire extra checkout of the repo reached the image. The
two entries are deliberately different shapes: anchored in .dockerignore,
where the `**/` form would also delete a legitimately named nested
directory, and unanchored in .gitignore, where a pattern already matches
at every depth. Anchoring leaves a gap where agents run in
subdirectories, stated in the vendored file itself. The second half is
the consequence of excluding .git: `git describe` in a build stage yields
an empty version without erroring, so the version is now computed on the
host and passed in.

Model: opus-5

prompts/EXISTING_REPO_CHECKLIST.md

@@ -24,9 +24,11 @@ with your task.
 - [ ] `LICENSE` file exists and matches the README
 - [ ] `REPO_POLICIES.md` exists and version date is current — fetch from
       `https://git.eeqj.de/sneak/prompts/raw/branch/main/prompts/REPO_POLICIES.md`
-- [ ] `.gitignore` is comprehensive (OS, editor, language artifacts, secrets) —
-      fetch from `https://git.eeqj.de/sneak/prompts/raw/branch/main/.gitignore`
-      if missing
+- [ ] `.gitignore` is comprehensive (OS, editor, agent scratch, language
+      artifacts, secrets) — fetch from
+      `https://git.eeqj.de/sneak/prompts/raw/branch/main/.gitignore` if missing.
+      An existing repo usually has a hand-written one that is never re-fetched,
+      so check the entries rather than the file's presence.
 - [ ] `.editorconfig` exists — fetch from
       `https://git.eeqj.de/sneak/prompts/raw/branch/main/.editorconfig`
 - [ ] `Dockerfile` and `.dockerignore` exist; Dockerfile runs `make check` as a
@@ -42,6 +44,17 @@ with your task.
       binaries, test binaries, coverage output), written root-anchored —
       `/myapp`, never `**/myapp`. An existing repo is where such a binary is
       likeliest to already be sitting in the build context, invisible to git.
+- [ ] `.claude/` is in `.gitignore` (unanchored) and `.claude` in
+      `.dockerignore` (anchored, no `**/` prefix). Agent worktrees are entire
+      checkouts of the repo, so they inflate the context by a multiple of it and
+      can copy another session's unreviewed work into an image layer. If agents
+      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, that version is computed on the
+      host and passed with `--build-arg VERSION=...` by `script/docker` and
+      `script/cibuild`, and no stage calls `git describe`. 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`