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

@@ -0,0 +1,4 @@
+# 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

@@ -21,6 +21,21 @@ fmt-check, and commit.
 
 # Completed Steps
 
+- 2026-10-04: `REPO_POLICIES.md` now says which `Dockerfile` stages run
+  `script/bootstrap` (issue 90). The gate phases and the build stage start from
+  their pinned base images and install what those images lack either inline, as
+  the canonical Go `Dockerfile` does for `git`, or by running
+  `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

prompts/NEW_REPO_CHECKLIST.md

@@ -124,8 +124,10 @@ are thin shims calling them. Model scripts:
 - [ ] `script/bootstrap` / `make bootstrap` — installs all dependencies,
       idempotently, assuming nothing (pkg manager detection nix/apt/brew/apk;
       node used if present, else pinned version via nvm from a hash-verified
-      archive; pinned yarn via corepack); Dockerfile runs it instead of inline
-      installs
+      archive; pinned yarn via corepack); a non-server repo's development
+      environment stage runs it instead of inline installs; a gate phase or the
+      build stage installs what its base image lacks either inline or by running
+      it
 - [ ] `script/setup` / `make setup` — readies a fresh clone: runs `bootstrap`,
       then `install-precommit`, plus repo-specific init
 - [ ] `script/test` / `make test` — `docker build --no-cache --target test .`,

prompts/REPO_POLICIES.md

@@ -104,10 +104,14 @@ style conventions are in separate documents:
   `lint` phase and a `test` phase, with the final stage depending on both so the
   image cannot be built unless they pass. For non-server repos the final stage
   brings up a development environment; for server repos it is the runtime image.
-  Dockerfiles install development prerequisites by running `script/bootstrap`
-  rather than duplicating installs inline; COPY `script/` and the dependency
-  manifests (`package.json` + `yarn.lock`, `go.mod` + `go.sum`, etc.) before
-  running it.
+  The gate phases and the build stage start from their pinned base images and
+  install what those images lack either inline, as the canonical Go `Dockerfile`
+  below does for `git`, or by running `script/bootstrap`, as the `prompts`
+  repo's own `Dockerfile` does for its yarn packages. The development
+  environment stage installs development prerequisites by running
+  `script/bootstrap` rather than duplicating its installs inline. A stage that
+  runs `script/bootstrap` COPYs `script/` and the dependency manifests
+  (`package.json` + `yarn.lock`, `go.mod` + `go.sum`, etc.) before running it.
 
 - **Linting and testing run in Docker, as phases of the `Dockerfile`.** There is
   no separate lint file. `script/lint` and `script/test` each build one phase