fix: keep pnpm's package store out of the release builder's clean-source check

The package-release job failed its clean-tree assertion in CI. Reproduced
as root in a container with a bind-mounted checkout, as the Gitea runner
does: after checkout the tree is clean; `pnpm install --frozen-lockfile`
then creates `.pnpm-store/` (17,272 untracked files) at the checkout's
root, because pnpm keeps its store on the node_modules file system and
the checkout is its own mount; `pnpm build` changes nothing. No tracked
file is ever modified.

- `.pnpm-store/` is ignored, like node_modules: a package cache, never
  source. The clean-commit requirement is unchanged for every tracked
  file and every untracked file Git does not ignore.
- The clean-tree failure now lists `git status --short` and
  `git diff --stat`.
- Nothing is built before the clean-source assertion: the gate no longer
  runs a global `pnpm build` first. The builder asserts a clean tree,
  builds, then fails if its build changed the tree, before packing. Core's
  verifier is built after both releases, for the consumer proof only.
- Every secondary offline install (the second worktree, the consumer
  module) uses the workspace install's own store explicitly, since pnpm
  picks a store per file system and those directories are on another.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

.gitignore

@@ -1,4 +1,8 @@
 node_modules/
+# pnpm's content-addressed package store. pnpm keeps it beside node_modules on the same file system,
+# so when the checkout is its own mount (a CI runner's workspace volume, a bind-mounted container),
+# `pnpm install` creates it here, at the checkout's root. A package cache, never source.
+.pnpm-store/
 .turbo/
 coverage/
 dist/