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

docs/trust-boundary/boundary.md

@@ -115,12 +115,15 @@ both sockets are `0666`, which is all a dialer needs.
 
 <!-- vale write-good.Passive = NO -->
 
-**Ownership is declared, not repaired.** The tmpfiles.d entry written by
-`SyncAgentTmpfiles` names the uid/gid directly. Don't add a chown
-alongside it: `d` re-applies on every boot _and_ every agent
-spawn/destroy, and reverts any ownership set afterwards the next time
-any agent changes — which is exactly how this dir spent a long time at
-`0777 root root` while a privileged chown appeared to be fixing it.
+**One mechanism creates it, one sets its owner.** Before every start,
+hive-priv creates the dir when missing (`EnsureAgentSocketDir`, `0751
+root:root`, no symlink followed) and leaves an existing one alone. The
+container's own activation (`hive-agent-user-migrate`) then chowns it to
+the agent user and sets `0751`. The container has no user namespace, so
+that uid is the host inode's owner. Don't add a host-side chown or chmod:
+two owners of one path revert each other. Until the container activates,
+the dir is `0751 root`: nothing but root can plant a socket in it, and a
+legacy root-run harness can still bind.
 
 <!-- vale write-good.Passive = YES -->
 
@@ -128,9 +131,11 @@ The mode is load-bearing, not cosmetic. Write permission on a
 _directory_ is what confers the right to unlink its entries, whoever owns
 them, and the sticky bit is the only thing that would restrain that (it
 isn't set here). A world-writable socket dir therefore lets anything
-able to reach the path delete an agent's socket and bind its own — and
-nginx reaches all of `/run/hive-agent` as a plain host path. Dropping
-`o=w` removes that permission rather than qualifying it.
+able to reach the path delete an agent's socket and bind its own. On the
+host, any non-root process whose `/run` is writable can do that, such as
+a login session or dnsmasq. nginx only dials: `ProtectSystem=strict` makes its view
+of `/run` read-only. Dropping `o=w` removes that permission rather than
+qualifying it.
 
 ⚠️ **The gateway's nginx and dnsmasq are host services, next to
 `hive-c0re`** (see `docs/networking/gateway.md`) — there is no namespace between