hive-priv: create agent socket dirs on start; drop hyperhive-agents.conf

/etc/tmpfiles.d/hyperhive-agents.conf was a boot-time backstop (#2290)
that pre-created every agent's bind sources. The start preamble already
creates them for every c0re-driven start, and on this host only hive-c0re
starts agent containers. The file was also the reason the socket dir's
owner had to be declared there, which is how it spent its life at
`0777 root root` whenever the uid could not be resolved (#4742).

- hive-priv gains `EnsureAgentSocketDir { name }`, called from
  `set_nspawn_flags` in every start path. It creates
  `/run/hive-agent/<name>` `0751 root:root` with mkdirat relative to an
  O_DIRECTORY|O_NOFOLLOW fd for the parent. An existing entry has to be a
  directory (fstatat AT_SYMLINK_NOFOLLOW); anything else is refused, and a
  directory is left alone. hive-c0re's own create_dir_all went: its /run
  is read-only under ProtectSystem=strict.
- The container's `hive-agent-user-migrate` activation chowns that dir to
  the agent user and sets 0751, the same way it already handles state/ and
  harness/. It refuses a symlink or non-directory there, since `test -d`
  and chmod follow links. No host-side passwd parse, and no window where
  the dir is world-writable.
- `/run/hyperhive/agents/<name>` stays created by hive-c0re itself
  (`ensure_agent_runtime_dir`). It holds the `mcp.sock` that hive-c0re
  binds as hive-core, so it must not become root- or agent-owned.
- The `/run/hive-agent` parent is declared in hive-priv.nix, `0755
  root:root`, instead of hive-gateway's hive-core rule. hive-priv is its
  only writer now, and hive-priv's ReadWritePaths needs it to exist.
- The manager start in `ensure_root_agent` now goes through
  `converge_start_preamble` + `start_with_fallback`. It was a bare start,
  so after a reboot the manager's bind sources existed only because of the
  tmpfiles file, and its limits drop-in did not exist at all.
- Removed: `sync_tmpfiles`, `agent_uid_gid` / `parse_passwd_uid_gid`,
  `priv_client::sync_agent_tmpfiles`, `AgentTmpfilesEntry`, the tmpfiles
  body builder and their tests, plus the three call sites.
- Legacy: hive-priv unlinks the file at every start, ignoring ENOENT.
  `SyncAgentTmpfiles` stays one release as a payload-ignoring variant that
  does the same unlink and returns Ok, for an older hive-c0re.

Salvaged from #4752: the boundary.md correction that nginx only dials,
because ProtectSystem=strict makes its /run read-only.

Behaviour change: a manual `nixos-container start h-<name>` right after a
reboot, before hive-c0re has started that agent, now fails on a missing
bind source instead of starting.

Closes #4742

hive-c0re/src/lifecycle/host_config.rs

@@ -445,14 +445,11 @@ async fn set_nspawn_flags(
     // gateway sees it. Subdir bind (not socket file) keeps the inode
     // visible after the harness unlinks a stale socket on rebind.
     // Applies to manager and sub-agents alike.
+    // hive-priv creates it (`/run` is read-only to hive-c0re) as `0751 root`
+    // and leaves an existing one alone; the container's activation hands it
+    // to the agent user. Don't chown or chmod it here.
     let socket_dir = crate::agent_sockets::agent_dir_for(agent_name);
-    std::fs::create_dir_all(&socket_dir)
-        .with_context(|| format!("create {}", socket_dir.display()))?;
-    // Ownership is NOT repaired here. The dir's owner + mode are declared by
-    // the tmpfiles.d entry (`SyncAgentTmpfiles`), which is the mechanism that
-    // re-applies on every boot and every spawn — so a chown made here was
-    // silently reverted the next time any agent was spawned or destroyed.
-    // This `create_dir_all` only covers the window before that sync lands.
+    crate::priv_client::ensure_agent_socket_dir(agent_name).await?;
     binds.push(BindMount {
         host_path: socket_dir.to_string_lossy().into_owned(),
         container_path: socket_dir.to_string_lossy().into_owned(),