Salvages #19795 (@ptichalouf, issue #19788).

Problem

docker/stage2-hook.sh does chown -R hermes:hermes "$HERMES_HOME" whenever
the runtime hermes UID doesn't match the directory owner (typically: on first
boot or when HERMES_UID is remapped). When $HERMES_HOME is a bind-mounted
host directory that contains files not managed by hermes, the recursive
chown silently rewrites every file's ownership — destroying host-side file
ownership permanently.

Fix

Targeted chown:

• Top-level $HERMES_HOME itself is chowned (not its contents), so hermes
  can mkdir new subdirs.
• Hermes-owned subdirs get recursive chown — these are the same dirs
  seeded by the s6-setuidgid mkdir -p block below
  (cron sessions logs hooks memories skills skins plans workspace home profiles).
• Everything else in $HERMES_HOME (host files, host subdirs) keeps its
  original ownership.

Validation

Built three images: baseline (origin/main), the salvage in this PR, and ran
the same chown E2E harness against each.

The E2E harness:

1. Creates a host dir with a pre-existing host-owned HOST_FILE.txt and a
   host-owned-subdir/note.txt
2. Mounts it as /opt/data and runs docker run -e HERMES_UID=12345 --version
3. Inspects post-run ownership from inside the volume (via root-privileged
   helper container — host UID can no longer stat into a 12345-owned dir)

Baseline (origin/main, current behavior)

✓ stage2 chown path triggered: [stage2] Fixing ownership of /opt/data to hermes (12345)
✗ HOST_FILE.txt UID changed (1000 → 12345) — BUG #19788 PRESENT
✗ host-owned-subdir/note.txt UID changed (1000 → 12345) — recursive chown leaked
✓ HOST_FILE.txt content preserved (only the metadata, not the bytes)
✓ $HERMES_HOME/cron owned by 12345 (the remapped hermes UID)

Result: 5 pass, 2 fail — bug reproduces cleanly

Salvage (this PR)

✓ stage2 chown path triggered: [stage2] Fixing ownership of /opt/data (targeted) to hermes (12345)
✓ HOST_FILE.txt UID preserved (1000 → 1000) — bug #19788 not present
✓ host-owned-subdir/note.txt UID preserved (1000 → 1000)
✓ HOST_FILE.txt content preserved verbatim
✓ $HERMES_HOME/cron owned by 12345 (the remapped hermes UID)
✓ top-level $HERMES_HOME chowned to 12345 (hermes can create new subdirs)

Result: 7 pass, 0 fail

Also confirmed via the standard image smoke battery (6/6 pass): --version,
hermes_cli import inside venv, _tui_need_npm_install: False, --tui
no-TTY exit, tools.lazy_deps imports, gcc still present.

Authorship

Original change by @ptichalouf in #19795. Their branch targeted
docker/entrypoint.sh (a deprecated shim since the s6-overlay rework — the
real cont-init logic moved to docker/stage2-hook.sh); retargeted the
fix accordingly. Preserved attribution via Co-authored-by:.

Closes #19795.
Fixes #19788.