hive-c0re: decide matrix token presence from metadata, not by reading it

hive-priv writes an agent's matrix-token 0600 and owned by the agent,
so hive-c0re, running as hive-core, cannot read it. The read-based
token-present guard in ensure_user_for therefore never fired, and every
matrix sweep (boot, every 30 minutes, every rebuild) re-minted each
agent's token through the appservice login and restarted its
hive-matrix-daemon.

Decide presence with a stat instead: a non-empty regular file counts as
present. hive-core can stat the file through the 0755 state dir.

Closes #4665

hive-c0re/src/matrix.rs

@@ -132,6 +132,15 @@ fn token_path(name: &hive_types::Ident) -> PathBuf {
     Coordinator::agent_notes_dir(name).join("matrix-token")
 }
 
+/// Whether an agent's token file is present: a non-empty regular file.
+/// Decided from metadata alone, because hive-priv writes the file 0600
+/// and owned by the agent, so `hive-core` cannot read it but can stat it
+/// through the 0755 state dir. A check that reads the file always
+/// fails here and makes every sweep re-mint the token.
+fn token_file_present(path: &std::path::Path) -> bool {
+    std::fs::metadata(path).is_ok_and(|m| m.is_file() && m.len() > 0)
+}
+
 /// Password file for the agent's matrix account. Stored OUTSIDE the
 /// purgeable `agent_state_root` tree so it survives `destroy --purge`
 /// and allows re-login recovery when the same agent name is re-spawned.
@@ -719,11 +728,7 @@ pub async fn ensure_user_for(client: &reqwest::Client, name: &str, as_token: &st
     use std::os::unix::fs::PermissionsExt;
     let agent = hive_types::Ident::parse(name)
         .map_err(|e| anyhow::anyhow!("invalid agent name {name:?}: {e}"))?;
-    let path = token_path(&agent);
-    if path.exists()
-        && let Ok(existing) = std::fs::read_to_string(&path)
-        && !existing.trim().is_empty()
-    {
+    if token_file_present(&token_path(&agent)) {
         tracing::debug!(%name, "matrix: token already present");
         return Ok(());
     }
@@ -2096,4 +2101,30 @@ mod tests {
         let err = extract_access_token(&body).unwrap_err();
         assert!(err.to_string().contains("missing access_token"));
     }
+
+    #[test]
+    fn token_file_present_only_for_a_non_empty_regular_file() {
+        let dir = tempfile::tempdir().expect("tempdir");
+        let token = dir.path().join("matrix-token");
+        assert!(!token_file_present(&token), "missing file");
+        std::fs::write(&token, "").unwrap();
+        assert!(!token_file_present(&token), "empty file");
+        std::fs::write(&token, "tok\n").unwrap();
+        assert!(token_file_present(&token), "non-empty file");
+        assert!(!token_file_present(dir.path()), "directory");
+    }
+
+    /// The sweep runs as `hive-core`, which cannot read the agent-owned
+    /// 0600 token file. `chmod 000` does not stop root, so this test uses
+    /// content that `read_to_string` rejects instead: the predicate must
+    /// still report the file as present, which holds only if it never
+    /// reads the content.
+    #[test]
+    fn token_file_present_does_not_read_the_content() {
+        let dir = tempfile::tempdir().expect("tempdir");
+        let token = dir.path().join("matrix-token");
+        std::fs::write(&token, [0xff, 0xfe]).unwrap();
+        assert!(std::fs::read_to_string(&token).is_err());
+        assert!(token_file_present(&token));
+    }
 }