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/s6-rc.d/dashboard/run

@@ -0,0 +1,30 @@
+#!/command/with-contenv sh
+# shellcheck shell=sh
+# Dashboard service. Always declared so s6 has a supervised slot; if
+# HERMES_DASHBOARD isn't set to a truthy value we sleep forever and do
+# nothing. See OQ3-A in the plan.
+
+case "${HERMES_DASHBOARD:-}" in
+    1|true|TRUE|True|yes|YES|Yes) ;;
+    *) exec sleep infinity ;;
+esac
+
+cd /opt/data
+# shellcheck disable=SC1091
+. /opt/hermes/.venv/bin/activate
+
+dash_host="${HERMES_DASHBOARD_HOST:-0.0.0.0}"
+dash_port="${HERMES_DASHBOARD_PORT:-9119}"
+
+# Binding to anything other than localhost requires --insecure — the
+# dashboard refuses otherwise because it exposes API keys. Inside a
+# container this is the expected deployment.
+insecure=""
+case "$dash_host" in
+    127.0.0.1|localhost) ;;
+    *) insecure="--insecure" ;;
+esac
+
+# shellcheck disable=SC2086  # word-splitting of $insecure is intentional
+exec s6-setuidgid hermes hermes dashboard \
+    --host "$dash_host" --port "$dash_port" --no-open $insecure

docker/s6-rc.d/dashboard/type

@@ -0,0 +1 @@
+longrun

docker/s6-rc.d/main-hermes/run

@@ -0,0 +1,27 @@
+#!/command/with-contenv sh
+# shellcheck shell=sh
+# Main hermes service.
+#
+# IMPORTANT — this is NOT how the user's CMD runs.
+#
+# We chose Architecture B from the plan: the container's CMD (the bare
+# command the user passes to `docker run <image> …`) runs as /init's
+# "main program" via Docker's CMD mechanism, NOT as an s6-supervised
+# service. This is the canonical s6-overlay pattern for "container
+# exits when the program exits" semantics, and it lets us preserve
+# every pre-s6 invocation contract (chat passthrough, sleep infinity,
+# bash, --tui) without re-implementing argument routing through
+# /run/s6/container_environment.
+#
+# So why does this service exist at all? Two reasons:
+#   1. s6-rc requires at least one user service for the "user" bundle
+#      to be valid. We can't ship an empty bundle.
+#   2. Future work may want to supervise a long-lived hermes process
+#      (e.g. for gateway-server containers); having the slot already
+#      wired in keeps that change small.
+#
+# For now this service is a no-op: it sleeps forever, doing nothing.
+# The dashboard runs as a real s6 service alongside it (see
+# ../dashboard/run) and per-profile gateways register dynamically via
+# /run/service/ at runtime (Phase 4).
+exec sleep infinity

docker/s6-rc.d/main-hermes/type

@@ -0,0 +1 @@
+longrun