migrate: an unreadable applied flake.nix fails that repo's migration instead of being overwritten

Only a missing file counts as 'marker absent'; any other read error (EACCES,
EIO, non-UTF-8) used to read as empty, so the agent's flake.nix was replaced
and committed.

hive-c0re/src/migrate.rs

@@ -168,19 +168,29 @@ async fn enumerate_agents() -> Result<Vec<hive_types::Ident>> {
 }
 
 async fn migrate_applied_repo(name: &str) -> Result<()> {
-    let dir = crate::paths::applied_dir(name);
+    migrate_applied_dir(name, &crate::paths::applied_dir(name)).await
+}
+
+/// Rewrites the applied repo at `dir` to the module-only flake. A missing
+/// `flake.nix` counts as "marker absent"; any other read error fails the
+/// migration so the agent's file is never overwritten unread.
+async fn migrate_applied_dir(name: &str, dir: &Path) -> Result<()> {
     if !dir.join(".git").exists() {
         return Ok(());
     }
     let flake_path = dir.join("flake.nix");
-    let cur = std::fs::read_to_string(&flake_path).unwrap_or_default();
+    let cur = match std::fs::read_to_string(&flake_path) {
+        Ok(cur) => cur,
+        Err(e) if e.kind() == std::io::ErrorKind::NotFound => String::new(),
+        Err(e) => return Err(e).with_context(|| format!("read {}", flake_path.display())),
+    };
     if cur.contains(MODULE_FLAKE_MARKER) {
         return Ok(());
     }
     let want = lifecycle::initial_flake_nix();
     std::fs::write(&flake_path, want).with_context(|| format!("write {}", flake_path.display()))?;
     raw_git(
-        &dir,
+        dir,
         &[
             "-c",
             "user.name=c0re",
@@ -192,7 +202,7 @@ async fn migrate_applied_repo(name: &str) -> Result<()> {
     )
     .await?;
     raw_git(
-        &dir,
+        dir,
         &[
             "-c",
             "user.name=c0re",
@@ -206,7 +216,7 @@ async fn migrate_applied_repo(name: &str) -> Result<()> {
     .await?;
     // Relocate deployed/0 to the migration commit so
     // setup_applied's existence check passes.
-    raw_git(&dir, &["tag", "-f", "deployed/0", "HEAD"]).await?;
+    raw_git(dir, &["tag", "-f", "deployed/0", "HEAD"]).await?;
     tracing::info!(%name, "migration: applied repo migrated to module-only flake");
     Ok(())
 }
@@ -241,3 +251,22 @@ async fn raw_git(dir: &Path, args: &[&str]) -> Result<()> {
     }
     Ok(())
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[tokio::test]
+    async fn unreadable_flake_is_left_untouched() {
+        let dir = tempfile::tempdir().expect("tempdir");
+        std::fs::create_dir(dir.path().join(".git")).expect("mkdir .git");
+        let flake = dir.path().join("flake.nix");
+        let bytes = [0xff_u8, 0xfe, b'x'];
+        std::fs::write(&flake, bytes).expect("write flake");
+
+        let res = migrate_applied_dir("t", dir.path()).await;
+
+        assert!(res.is_err(), "non-UTF-8 flake.nix must fail the migration");
+        assert_eq!(std::fs::read(&flake).expect("read back"), bytes);
+    }
+}