lifecycle: make the agent-uid lookup say which failure it hit

`agent_uid_gid` returns `None` for all 13 agents on every sync, and the
tmpfiles caller answers that `None` by writing `d /run/hive-agent/<name>
0777 root root` instead of `0751 <uid> <gid>` — a world-writable socket
dir, which docs/trust-boundary/boundary.md spells out as letting anything
that can reach the path unlink an agent's socket and bind its own.

Which failure fires could not be determined, because the read used
`.ok()?` and collapsed every io::Error into the same `None` a missing
user produces. None of the three causes the doc comment enumerated (not
built yet, unparseable, missing user) fits 13 long-lived containers whose
agent user demonstrably exists — from inside one:

    srw-rw-rw- atlas atlas /run/hive-agent/atlas/agent.sock

So the live cause was outside the documented set and unidentifiable. Both
`None` arms now log, with the path and the error.

Splitting the pure parser out to make it testable surfaced a second,
narrower bug. The old scan used `?` on the field reads, and those are
only reached once the name matches — so an unusable row *for the wanted
user* returned `None` from the whole function instead of skipping, hiding
a usable entry below it. (Rows for other users were always skipped fine:
`split(':')` always yields at least one item, so the first `?` could not
fire.) It now skips unusable rows and keeps looking.

Does not pre-empt #3047, which removes the lookup entirely and stays
blocked on #3998; this only makes the lookup honest about failing while
it exists.

Closes #4197.

hive-c0re/src/lifecycle/mod.rs

@@ -178,9 +178,10 @@ pub fn network_isolation_from_env() -> Result<hive_priv_sock::NetworkIsolation>
 }
 
 /// Read the agent user's `(uid, gid)` from the container's nixos-managed
-/// `/etc/passwd`. Returns `None` when the container hasn't been built
-/// yet, the passwd file is unparseable, or the agent user is missing
-/// (e.g. legacy container that still runs as root).
+/// `/etc/passwd`. Returns `None` when the passwd file cannot be read (not
+/// built yet, or this process cannot reach the path), or when it is read
+/// but holds no usable entry for the agent (missing user, e.g. a legacy
+/// container that still runs as root).
 ///
 /// Used by `forge` + `matrix` after writing per-agent state files so
 /// the bind-mounted host file ends up readable by the agent user
@@ -194,20 +195,56 @@ pub fn network_isolation_from_env() -> Result<hive_priv_sock::NetworkIsolation>
 ///     `PrivateUsers`), so the uid is directly usable in host-side
 ///     `chown(2)`.
 ///   - Best-effort: caller treats `None` as "skip the chown".
+///   - ⚠️ Every `None` is logged with which of those cases produced it: the
+///     tmpfiles caller answers `None` by declaring the agent's socket dir
+///     `0777`, and a fallback that widens a directory has to say why it fired.
 #[must_use]
 pub fn agent_uid_gid(agent_name: &str) -> Option<(u32, u32)> {
     let container = container_name(agent_name);
     let passwd_path = format!("/var/lib/nixos-containers/{container}/etc/passwd");
-    let content = std::fs::read_to_string(&passwd_path).ok()?;
+    let content = match std::fs::read_to_string(&passwd_path) {
+        Ok(content) => content,
+        Err(e) => {
+            tracing::warn!(
+                agent = %agent_name,
+                path = %passwd_path,
+                error = ?e,
+                "agent_uid_gid: cannot read the container's passwd"
+            );
+            return None;
+        }
+    };
+    let ids = parse_passwd_uid_gid(&content, agent_name);
+    if ids.is_none() {
+        tracing::warn!(
+            agent = %agent_name,
+            path = %passwd_path,
+            lines = content.lines().count(),
+            "agent_uid_gid: passwd read but no usable entry for this agent"
+        );
+    }
+    ids
+}
+
+/// Find `user`'s `(uid, gid)` in a `passwd(5)` body.
+///
+/// Separate from the read so it can be tested at all — the caller's half is a
+/// host path no test can stand up.
+fn parse_passwd_uid_gid(content: &str, user: &str) -> Option<(u32, u32)> {
     for line in content.lines() {
         let mut parts = line.split(':');
-        let user = parts.next()?;
-        if user != agent_name {
+        // Skip, never abort: an unusable row for this same user must not hide
+        // a usable one below it.
+        if parts.next() != Some(user) {
             continue;
         }
-        let _ = parts.next()?; // x (password placeholder)
-        let uid: u32 = parts.next()?.parse().ok()?;
-        let gid: u32 = parts.next()?.parse().ok()?;
+        let _password = parts.next();
+        let (Some(uid), Some(gid)) = (parts.next(), parts.next()) else {
+            continue;
+        };
+        let (Ok(uid), Ok(gid)) = (uid.parse(), gid.parse()) else {
+            continue;
+        };
         return Some((uid, gid));
     }
     None