refactor: remove the dormant slot privilege seam (hal0-slotctl + euid routing)

[thinmintdev]

Summary

Follow-up to #953 (hardened mode removed). Now that hal0-api always runs as root, the slot privilege seam that existed solely to let an unprivileged daemon write per-slot units + run systemctl is dead code. This removes it — behavior‑preserving, since the kept path is exactly the euid == 0 (root) branch that already ran in production.

Removed

• ContainerProvider._privileged() and the two if os.geteuid() == 0 … else <sudo -n hal0-slotctl …> branches in container.py (kept the root branch; dropped the _HAL0_SLOTCTL constant).
• The hal0-slotctl wrapper (installer/wrappers/hal0-slotctl) + its sudoers grant (packaging/sudoers/hal0-slotctl) + the install.sh block that wrote them — replaced with an upgrade cleanup that deletes any stale copies left by an older hardened install.
• The sudo -n fallback in installer.py:_privileged_systemctl_argv (same dormant euid pattern; comfyui restart now goes direct).
• The obsolete seam test (tests/providers/test_container_privileged_seam.py) and the conftest.py autouse geteuid==0 fixture (no longer needed — container.py never calls geteuid now).

Kept (intentionally)

• src/hal0/install/perms.py — still used by hal0 doctor to audit root-only + agent (#843) ownership. Its stale "seam is the prerequisite" docstring is updated; its now-unwired non-root branches are left as a candidate for a later cleanup alongside doctor.
• The agent env seam (hal0-agentenv, run-as-hal0.sh) — agents/Hermes still run as hal0, so this stays.

Verification

• 289 targeted tests pass (tests/providers + tests/install/test_perms.py + tests/api/test_installer_routes.py).
• ruff check + ruff format --check clean; bash -n installer/install.sh clean.
• Net −314 lines.

Test plan

• CI green.
• On a root-mode box, slot load/unload still writes the unit + daemon-reload/restart directly (unchanged path).
• Upgrading a previously-hardened box removes the stale hal0-slotctl + /etc/sudoers.d/hal0-slotctl.

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