fix(#2733): write the agent pause marker via hive-priv

`Coordinator::set_paused` wrote the marker directly with `std::fs::write`
from hive-c0re, which runs as the unprivileged `hive-core` user. The
agent's harness dir is chowned to the agent user on every container boot
(`user.nix`'s activation chown), mode 0755 — so hive-core can stat the
marker but gets EACCES creating or unlinking it. Pause therefore only
ever worked on an agent that had never booted; the read side works
because a stat needs traverse, not write, which is why the paused pill
and `is_paused` looked healthy.

Route both directions through hive-priv, the root helper that already
owns the other writes into agent-owned directories:

- `PrivRequest::SetAgentPaused { agent_name, paused }`, with the marker
  filename constant moved to hive-priv-sock. That is the narrowest crate
  all three sides share (hive-priv deliberately does not depend on
  hive-sh4re, which re-exports it for the in-container resolver). A
  private copy on any one side would break pause silently, since every
  reader just sees "no marker".
- `write_agent_state_file` generalised to `write_agent_dir_file`, taking
  the target directory: `state/` and `harness/` are both agent-owned,
  which is the same reason both need root.
- resume unlinks via `remove_file`, which acts on the leaf and never
  follows a symlink — an agent could otherwise plant a link at the
  marker path and have root delete an arbitrary file.

`Coordinator::set_paused` becomes an async round-trip; its three call
sites were already async. Both directions stay idempotent because the
dashboard toggle and `hivectl pause|resume` fire without reading the
current state first.

Cargo.lock

@@ -1780,6 +1780,7 @@ name = "hive-sh4re"
 version = "0.1.0"
 dependencies = [
  "chrono",
+ "hive-priv-sock",
  "hive-types",
  "schemars",
  "serde",