Say that a build from a linked worktree needs the version passed in (closes #111)

A plain `docker build .` from a checkout whose `.git` is a file (a linked worktree, or a repository checked out as a submodule) stops at the version check of the canonical `Dockerfile`: that file points to a git directory outside the build context, so `git describe` prints nothing. The policy said a plain build must succeed and did not name this case.

`prompts/REPO_POLICIES.md` and both checklists now say, right after that rule, that such a checkout is the exception and is given its version with `--build-arg VERSION=...`, as `script/docker` and `script/cibuild` already do. The check is unchanged.

Model: opus-5-5

TODO.md

@@ -21,6 +21,14 @@ fmt-check, and commit.
 
 # Completed Steps
 
+- 2026-10-06: `REPO_POLICIES.md` and both checklists now say that a checkout
+  whose `.git` is a file, a linked worktree or a repository checked out as a
+  submodule, is the exception to a plain `docker build .` succeeding (issue
+  111): that file points to a git directory outside the build context, so the
+  build cannot read the version and the version check in the canonical
+  `Dockerfile` stops it. Such a build is given its version with
+  `--build-arg VERSION=...`, as `script/docker` and `script/cibuild` already do.
+  The check itself is unchanged.
 - 2026-10-06: The canonical `.gitignore` and `.editorconfig` now each end with a
   comment saying the repository's own entries go below it and a re-vendor keeps
   them (issue 103, which took in issue 104), as `.dockerignore`'s header already