fix(#3900): overlay walker skips non-regular files — a repo-root socket no longer fails 32 tests (#4062)

* test(#3900): a unix socket under the repo root must not break the overlay (failing first)

* fix(#3900): the overlay walker skips non-regular files (sockets, FIFOs, device nodes)

The walker classified every not-a-directory entry as a file, so a unix
socket anywhere under the repo root (an MCP server, a language-server
daemon, a watcher) made copyFileSync throw ENXIO — the reporter
measured 32 local test failures across 6 files, all in before() hooks,
none about the code under test; a FIFO would block until a writer
appeared. The hard-link path does not save it: the overlay sits under
tmpfs, so cross-device EXDEV always falls through to the copy.

One guard at the classification site (srcStat.isFile() before the
copy/link branches) — sockets, FIFOs, and device nodes are not
repository content; CI never saw this because a fresh clone has no
working-tree daemons, which is exactly why it ate local debugging
time. Also adds opts.root (test-only root override, default REPO_ROOT)
so the regression tests build the overlay from a tiny synthetic root
in milliseconds instead of paying a full repo walk.

* chore(#3900): changeset fragment (pr number backfilled after PR creation)

* fix(#3900): review fold-ins — sibling cold-fixture guard, changeset format, root doc

buildColdInstallTree enumerated the live repo's hooks/ with a NAME-only
filter — the same class #3900 documents: a daemon socket or FIFO inside
hooks/ would reach cpSync and throw ENXIO (or block). Dirent type guard
added. The changeset gains the mandated bold user-visible lead, and
opts.root is documented at its definition (mirroring cold-runtime-lib-
fixture's repoRoot convention).

* chore(#3900): backfill changeset PR number (4062)

---------

Co-authored-by: sim <sim@local>

tests/helpers/cold-runtime-lib-fixture.cjs

@@ -77,6 +77,10 @@ function buildColdInstallTree(opts = {}) {
   fs.mkdirSync(hooksDestDir, { recursive: true });
   for (const entry of fs.readdirSync(path.join(repoRoot, 'hooks'), { withFileTypes: true })) {
     if (!shouldCopyHookEntry(entry.name)) continue;
+    // #3900 (same-class guard, found in review): the live repo tree is walked
+    // here — a daemon's socket/FIFO inside hooks/ would reach cpSync and
+    // throw ENXIO (a FIFO would block). Dirent type check, not name-only.
+    if (!entry.isFile() && !entry.isDirectory()) continue;
     fs.cpSync(path.join(repoRoot, 'hooks', entry.name), path.join(hooksDestDir, entry.name), {
       recursive: true,
     });