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/CODE_STYLEGUIDE_GO.md

@@ -49,7 +49,14 @@ last_modified: 2026-09-08
     ```
 
     ```make
-    VERSION := $(shell git describe --always --dirty)
+    # ?= rather than := because this `$(shell git describe ...)` is only
+    # correct on the host: `.dockerignore` excludes `.git`, so evaluated
+    # inside a build stage it expands to the empty string without failing
+    # and the binary reports no version. The version is computed on the
+    # host by `script/docker` / `script/cibuild` and passed with
+    # `--build-arg VERSION=...`; where a build stage invokes make,
+    # `ARG VERSION` puts it in the environment and `?=` defers to it.
+    VERSION ?= $(shell git describe --always --dirty)
     BUILDARCH := $(shell uname -m)
 
     GOLDFLAGS += -X main.Version=$(VERSION)