Say which Dockerfile stages run script/bootstrap (closes #90)

The bullet in `prompts/REPO_POLICIES.md` that requires a `Dockerfile` said every Dockerfile installs its prerequisites by running `script/bootstrap`, while the canonical Go `Dockerfile` never runs it.

The bullet now says the gate phases and the build stage start from their pinned base images and install what those images lack either inline, as the Go example does for `git`, or by running `script/bootstrap`, as the `prompts` repo's own `Dockerfile` does for its yarn packages. The development environment stage, the final stage of a non-server repo, runs `script/bootstrap`. The new repo checklist says the same.

Model: opus-5-5

.gitattributes

@@ -1,4 +0,0 @@
-# Every PR adds an entry at the top of TODO.md's Completed Steps; union keeps
-# both sides instead of conflicting. Git never reports a conflict here: read
-# the merged entries after every merge or rebase.
-TODO.md merge=union

TODO.md

@@ -28,14 +28,6 @@ fmt-check, and commit.
   `script/bootstrap`, as this repo's own `Dockerfile` does for its yarn
   packages. The development environment stage, the final stage of a non-server
   repo, runs `script/bootstrap`. The new repo checklist says the same.
-- 2026-10-04: Added a root `.gitattributes` that merges `TODO.md` with git's
-  union merge (issue 98), so two branches that each add an entry at the top of
-  Completed Steps merge without a conflict. Git now never reports a conflict in
-  `TODO.md`: a real conflict elsewhere keeps both versions of the line, and when
-  two new entries share an identical line, one is inserted into the middle of
-  the other, which a rebase can do to an entry already on `next`. Read the
-  merged entries after every merge or rebase. This applies to this repository
-  only; no canonical file changed.
 - 2026-10-04: The canonical `.dockerignore` now also keeps out the git `config`
   of a submodule that keeps its own `.git` directory, which still reached the
   image (issue 88): both git patterns now carry the `**/` prefix. A submodule