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/lifecycle/tests.rs

@@ -283,50 +283,6 @@ fn an_unknown_state_is_never_reported_as_failed() {
     }
 }
 
-/// A malformed row for the wanted user must not end the scan.
-///
-/// The old implementation used `?` on the field reads, which are only reached
-/// once the name matches — so a short or unparseable row *for this agent*
-/// returned `None` from the whole function and hid a usable entry below it.
-/// The caller answers that `None` by declaring the socket dir `0777`.
-#[test]
-fn passwd_parse_keeps_scanning_past_a_malformed_row_for_the_same_user() {
-    // The two `atlas` rows above the real one are what matters: a malformed
-    // row for the SAME user is what used to end the scan, so the usable entry
-    // below it was never reached. Rows for other users were always skipped
-    // fine. Both unusable shapes are here because they leave the parser by
-    // different arms — missing fields, and fields that will not parse — and a
-    // mutation run showed the second arm untested when only the first was.
-    let body = "\
-root:x:0:0:System administrator:/root:/bin/sh
-# a comment line, not a passwd entry
-atlas:x
-atlas:x:notanumber:994::/:/bin/sh
-atlas:x:1000:994:atlas:/home/atlas:/bin/sh
-";
-    assert_eq!(parse_passwd_uid_gid(body, "atlas"), Some((1000, 994)));
-    // Control: the same body must NOT answer for a user it does not carry,
-    // or the assertion above is satisfied by any parse at all.
-    assert_eq!(parse_passwd_uid_gid(body, "nobody-here"), None);
-}
-
-/// A matching name with unusable ids is not a match. Without this the parser
-/// could return a partly-parsed row and the caller would chown to it.
-#[test]
-fn passwd_parse_rejects_unusable_ids() {
-    assert_eq!(
-        parse_passwd_uid_gid("atlas:x:notanumber:994::/:/bin/sh", "atlas"),
-        None
-    );
-    assert_eq!(parse_passwd_uid_gid("atlas:x:1000", "atlas"), None);
-    assert_eq!(parse_passwd_uid_gid("", "atlas"), None);
-    // Presence control for the three absences above.
-    assert_eq!(
-        parse_passwd_uid_gid("atlas:x:1000:994::/:/bin/sh", "atlas"),
-        Some((1000, 994))
-    );
-}
-
 /// A failed `nixos-container destroy` whose container is still listed fails,
 /// and keeps hive-priv's error in the chain. The destroy DAG gates the state
 /// purge on this call, so this is the case that must not read as success.