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/priv_client.rs

@@ -8,8 +8,8 @@
 
 use anyhow::{Context as _, Result, bail};
 use hive_priv_sock::{
-    AgentTmpfilesEntry, BindMount, CredentialMount, InfraAction, InfraContainer, JournalQuery,
-    NetworkIsolation, PRIV_SOCK, PrivEvent, PrivRequest, PrivResponse, PrivStream,
+    BindMount, CredentialMount, InfraAction, InfraContainer, JournalQuery, NetworkIsolation,
+    PRIV_SOCK, PrivEvent, PrivRequest, PrivResponse, PrivStream,
 };
 use std::os::fd::{AsRawFd as _, OwnedFd, RawFd};
 
@@ -306,6 +306,14 @@ pub async fn write_nspawn_flags(
     .await?)
 }
 
+/// See [`PrivRequest::EnsureAgentSocketDir`].
+pub async fn ensure_agent_socket_dir(name: &str) -> Result<()> {
+    ok(call(&PrivRequest::EnsureAgentSocketDir {
+        name: name.to_owned(),
+    })
+    .await?)
+}
+
 pub async fn write_resource_limits(
     container: &str,
     memory_max: &str,
@@ -616,22 +624,6 @@ pub async fn send_agent_snapshot_to_file(
     Ok(stdout)
 }
 
-/// Write `/etc/tmpfiles.d/hyperhive-agents.conf` for `agents` and immediately
-/// apply it with `systemd-tmpfiles --create`. Each entry carries the agent's
-/// container uid/gid so the socket dir's ownership is *declared* here rather
-/// than corrected afterwards. See [`PrivRequest::SyncAgentTmpfiles`].
-///
-/// # Errors
-///
-/// Returns an error if the priv socket call fails, if any agent name is
-/// invalid, or if `systemd-tmpfiles --create` exits non-zero.
-pub async fn sync_agent_tmpfiles(agents: &[AgentTmpfilesEntry]) -> Result<()> {
-    ok(call(&PrivRequest::SyncAgentTmpfiles {
-        agents: agents.to_vec(),
-    })
-    .await?)
-}
-
 /// Parse `(referenced, exclusive)` bytes from `btrfs qgroup show -f --raw`
 /// output (a qgroup row is `<id-with-slash> <rfer> <excl> …`).
 ///