`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.
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