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/module-eval/agent-user.nix

@@ -0,0 +1,49 @@
+# `checks.module-eval-agent-user` — see ./lib.nix for the shared
+# rationale (why this suite exists, naming convention, "evaluates
+# not executes").
+{
+  pkgs,
+  lib,
+  self,
+  nixosSystem,
+}:
+let
+  inherit
+    (import ./lib.nix {
+      inherit
+        pkgs
+        lib
+        self
+        nixosSystem
+        ;
+    })
+    agentWith
+    runGroup
+    ;
+
+  # A named agent, so the rendered script carries a name no default supplies.
+  migrate =
+    (agentWith { services.hyperhive.agent.user.name = "iris"; })
+    .system.activationScripts.hive-agent-user-migrate.text;
+  cases = [
+    {
+      # hive-priv creates the host socket dir `0751 root` and never chowns it,
+      # so this activation is the only thing that lets the harness bind there.
+      name = "the agent's activation hands its socket dir to the agent user at 0751";
+      ok =
+        lib.hasInfix "userName=${lib.escapeShellArg "iris"}" migrate
+        && lib.hasInfix ''socketDir="/run/hive-agent/$userName"'' migrate
+        && lib.hasInfix ''chown -h "$userName:$userName" "$socketDir"'' migrate
+        && lib.hasInfix ''chmod 0751 "$socketDir"'' migrate;
+    }
+    {
+      # `test -d` and `chmod` follow symlinks, so without this refusal ahead
+      # of the chown branch a link at the socket dir is chmodded at its target.
+      name = "the agent's activation refuses a symlinked or non-directory socket dir";
+      ok =
+        lib.hasInfix ''if [ -L "$socketDir" ] || { [ -e "$socketDir" ] && [ ! -d "$socketDir" ]; }; then'' migrate
+        && lib.hasInfix ''elif [ -d "$socketDir" ]; then'' migrate;
+    }
+  ];
+in
+runGroup "agent-user" cases