Part A of moving hive-ci runner registration off the boot-critical path.
Adds a root-side hive-priv op that writes the runner registration token to
the host env-file /run/hive-ci/runner-token (in-place, preserving the inode
the container bind-mounts) and restarts the in-container gitea-runner-hive
unit. The forge admin token stays in hive-c0re; only the registration token
reaches the host env-file the container mounts read-only. The c0re-side
caller (ensure_ci_runner_registered) + the nix boot-path change land next on
this branch.
Split the priv-socket wire types (PrivRequest/PrivResponse/PrivEvent and
friends) out of hive-sh4re into their own hive-priv-sock crate, mirroring
the existing hive-host-sock split. hive-priv — the root-privileged
helper — now depends on just this narrow protocol crate instead of the
much larger daemon-shared crate, shrinking its dependency surface and
making the privsep boundary easier to audit. No server/client
implementation lives here, only the wire contract; hive-c0re still
depends on hive-sh4re directly for everything else.
