Unexpected git failures now surface as Error::Git rather than
Error::NotFound, and UTF-8 decoding failures use Error::Io with
InvalidData rather than Error::NotFound.

Assisted-by: GLM-5.1 via pi

diff --git a/src/error.rs b/src/error.rs
index 7ec6b81..7f4940c 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -14,6 +14,9 @@ pub enum Error {
 
     #[error("fennel error: {0}")]
     Fennel(String),
+
+    #[error("git error: {0}")]
+    Git(String),
 }
 
 pub type Result<T> = std::result::Result<T, Error>;
diff --git a/src/quire.rs b/src/quire.rs
index 0828b55..fd552d2 100644
--- a/src/quire.rs
+++ b/src/quire.rs
@@ -71,13 +71,17 @@ impl Repo {
                 return Ok(RepoConfig::default());
             }
             // Unexpected git error.
-            return Err(crate::Error::NotFound(format!(
+            return Err(crate::Error::Git(format!(
                 "failed to read HEAD:.quire/config.fnl: {stderr}"
             )));
         }
 
-        let source = String::from_utf8(output.stdout)
-            .map_err(|e| crate::Error::NotFound(format!("config is not valid UTF-8: {e}")))?;
+        let source = String::from_utf8(output.stdout).map_err(|e| {
+            crate::Error::Io(std::io::Error::new(
+                std::io::ErrorKind::InvalidData,
+                format!("config is not valid UTF-8: {e}"),
+            ))
+        })?;
 
         let fennel = Fennel::new().map_err(|e| crate::Error::Fennel(e.to_string()))?;
         fennel