Add user-slice plumbing for ascend/asctl

[Copilot]

This adds the basic plumbing for per-user ascend instances, similar to systemctl --user. ascend now resolves into a user slice automatically when started as a non-root Unix user, and asctl can target user slices explicitly.

• User-slice resolution

  • Added shared slice resolution logic in src/common/slices.lua
  • Non-root Unix ascend defaults now move from system paths to per-user paths:
    • services under XDG_CONFIG_HOME / ~/.config
    • logs under XDG_STATE_HOME / ~/.local/state
    • IPC socket under XDG_RUNTIME_DIR when available
  • System-scope defaults remain unchanged for root/system operation

• CLI targeting

  • Added --user / -u support to asctl
  • asctl --user targets the current user slice
  • asctl --user=<name> targets another user’s slice via a user-specific socket path

• Process model changes

  • When ascend is running in user-slice mode, per-service user overrides are ignored
  • This keeps the process model aligned with a user-owned slice instead of mixing per-service identity changes

• Docs and coverage

  • Updated the README to document user-slice behavior and asctl --user
  • Added focused isolation tests for:
    • user-slice default path/socket resolution
    • explicit cross-user asctl socket targeting
    • ignoring service-level user overrides in user-slice mode

Example:

# current user's ascend instance
asctl --user list

# another user's ascend instance
asctl --user=alice status my-service