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

nix/host-modules/hive-priv.nix

@@ -20,6 +20,11 @@ let
 in
 {
   config = lib.mkIf config.services.hyperhive.deploy.hive-controller.enable {
+    # The parent of every agent socket dir. hive-priv is its only writer
+    # (`EnsureAgentSocketDir`), and its unit lists it in `ReadWritePaths`,
+    # which fails the unit when the path is missing.
+    systemd.tmpfiles.rules = [ "d /run/hive-agent 0755 root root - -" ];
+
     # Socket unit for hive-priv — the narrow root helper that executes
     # privileged operations on behalf of hive-c0re. Systemd creates and
     # holds `/run/hive/priv.sock` before the first connection arrives.
@@ -108,8 +113,9 @@ in
         # hive-priv must write to at runtime. Each is a confirmed hive-priv
         # write that EROFSes (os error 30) without its carve-out:
         #   /etc/nixos-containers     — <container>.conf (bind mounts, nspawn flags)
-        #   /etc/tmpfiles.d           — sync_tmpfiles' hyperhive-agents.conf write
-        #   /run/hive-agent           — chown/chmod per-agent socket dirs
+        #   /etc/tmpfiles.d           — unlinks the legacy hyperhive-agents.conf;
+        #                               drop once every host has run it
+        #   /run/hive-agent           — creates per-agent socket dirs
         #   /run/systemd              — container@ drop-ins + machined state
         #   /run/lock                 — nixos-container's create/destroy lock file
         #   /run/hive-ci              — register_ci_runner's runner-token write