feat(docker)!: replace tini with s6-overlay as PID 1

BREAKING CHANGE: the container ENTRYPOINT is now /init (s6-overlay)
instead of /usr/bin/tini. Main hermes runs as the container CMD with
TTY inherited (preserving --tui), dashboard runs as a supervised s6-rc
service (HERMES_DASHBOARD=1 starts it; crashes auto-restart), and the
ground is laid for per-profile gateway supervision (Phase 3+4).

All five pre-s6 docker run invocation patterns continue to work
identically — verified by the Phase 0 docker harness:

  docker run <image>                  → `hermes` with no args
  docker run <image> chat -q "..."    → `hermes chat -q ...` passthrough
  docker run <image> sleep infinity   → `sleep infinity` direct
  docker run <image> bash             → interactive bash
  docker run -it <image> --tui        → interactive Ink TUI

Phase 2 harness result: 12 passed, 2 xfailed (Phase 4 target). Hadolint
+ shellcheck pass cleanly.

Architecture pivot from plan v3 (documented in main-hermes/run header):
the plan called for main hermes to be an s6-supervised service, but
two real s6-overlay v3 mechanics blocked that — cont-init.d scripts
receive no arguments (CMD args are not visible to stage2-hook), and
`/run/s6/basedir/bin/halt` after writing the exit code did not
propagate the desired exit code (container exits 143). We use the
s6-overlay-native CMD pattern instead: main-wrapper.sh is the
container's main program (ENTRYPOINT prepends it so leading-dash
args like --version aren't intercepted by /init), exec's the final
program with stdin/stdout/stderr inherited, and the program's exit
code becomes the container exit code. main-hermes is now a no-op
`sleep infinity` slot kept for future supervised-gateway-container
modes. This trades "supervised restart of main hermes" for arg-
parity with the pre-s6 contract — main hermes was already unsupervised
under tini, so we lose nothing functional. Dashboard supervision is
the only new guarantee added by this phase.

Files added:
  docker/main-wrapper.sh           # arg routing + s6-setuidgid drop
  docker/stage2-hook.sh            # gosu-equivalent + chown + seed
  docker/s6-rc.d/main-hermes/{type,run,dependencies.d/base}
  docker/s6-rc.d/dashboard/{type,run,dependencies.d/base}
  docker/s6-rc.d/user/contents.d/{main-hermes,dashboard}

Files changed:
  Dockerfile: tini → s6-overlay install + ENTRYPOINT flip + service wiring
  docker/entrypoint.sh: thin shim to stage2-hook.sh for back-compat
  tests/docker/test_dashboard.py: add test_dashboard_restarts_after_crash

Refs: docs/plans/2026-05-07-s6-overlay-dynamic-subagent-gateways.md

docker/main-wrapper.sh

@@ -0,0 +1,30 @@
+#!/bin/sh
+# /opt/hermes/docker/main-wrapper.sh — wraps the container's CMD with
+# the same argument-routing logic the pre-s6 entrypoint.sh used. Runs
+# as /init's "main program" (Docker CMD) so it inherits stdin/stdout/
+# stderr from the container.
+#
+# Routing:
+#   no args                       → exec `hermes` (the default)
+#   first arg is an executable    → exec it directly (sleep, bash, sh, …)
+#   first arg is anything else    → exec `hermes <args>` (subcommand passthrough)
+#
+# We drop to the hermes user via `s6-setuidgid` — running as that
+# user matches the pre-s6 contract (gosu drop).
+set -e
+
+cd /opt/data
+# shellcheck disable=SC1091
+. /opt/hermes/.venv/bin/activate
+
+if [ $# -eq 0 ]; then
+    exec s6-setuidgid hermes hermes
+fi
+
+if command -v "$1" >/dev/null 2>&1; then
+    # Bare executable — pass through directly.
+    exec s6-setuidgid hermes "$@"
+fi
+
+# Hermes subcommand pass-through.
+exec s6-setuidgid hermes hermes "$@"