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

README.md

@@ -123,10 +123,13 @@ alpine. We provide:
 - `script/check` — run all checks: `test`, `lint`, `fmt-check` (our own
   extension)
 - `script/docker` — build the Docker image, tagged via `script/projectname`
-  (byte-identical across repos); builds with `--no-cache`, like `script/cibuild`
-- `script/cibuild` — cd to the repo root and `docker build --no-cache .` (what
-  CI runs; the image build runs `script/check`, and `--no-cache` is what stops
-  Docker serving those checks from cache on an unchanged tree)
+  (byte-identical across repos); same `--no-cache` and `VERSION` as
+  `script/cibuild`
+- `script/cibuild` — cd to the repo root, compute `version` from `git describe`,
+  then `docker build --no-cache --build-arg VERSION="$version" .` (what CI runs;
+  the image build runs `script/check`, `--no-cache` is what stops Docker serving
+  those checks from cache on an unchanged tree, and the version is computed on
+  the host because `.dockerignore` excludes `.git`)
 - `script/precommit` — run by the git pre-commit hook (our own extension); calls
   `script/check`
 - `script/install-precommit` — installs the git pre-commit hook (our own