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/agent-modules/user.nix

@@ -209,6 +209,20 @@ in
       if [ -d "/agents/$userName/harness" ]; then
         chown -hR "$userName:$userName" "/agents/$userName/harness" 2>/dev/null || true
       fi
+      # The socket dir is bind-mounted from the host, where hive-priv creates
+      # it `0751 root`. The harness binds its sockets here as this user; the
+      # 0751 is what lets hive-c0re and nginx dial them without listing
+      # (docs/trust-boundary/boundary.md, "the per-agent socket dir"). Not
+      # recursive: only the harness writes inside it. `test -d` and `chmod`
+      # follow symlinks, so a link here is refused rather than handed over.
+      socketDir="/run/hive-agent/$userName"
+      if [ -L "$socketDir" ] || { [ -e "$socketDir" ] && [ ! -d "$socketDir" ]; }; then
+        echo "hive-agent-user-migrate: $socketDir is a symlink or not a directory; not handing it to $userName" >&2
+      elif [ -d "$socketDir" ]; then
+        chown -h "$userName:$userName" "$socketDir" \
+          && chmod 0751 "$socketDir" \
+          || echo "hive-agent-user-migrate: could not hand $socketDir to $userName; the harness cannot bind its sockets" >&2
+      fi
       # The proposed-config repo is RW-mounted into the editing agent and
       # owned by it; hive-c0re only pulls from it. Heal
       # it to this user too — same as state/harness. In an agent's own