fix(git): use repo.default_branch instead of HEAD to avoid master/branch mismatch

- Add #[serde(default)] to MergeAnalysisQuery.their_oid and MergeRefAnalysisQuery fields
  since they come from path parameters, not query strings
- git_readme/git_commit_count/git_contributors: default to repo.default_branch
  instead of HEAD to avoid errors when HEAD points to refs/heads/master but
  the actual default branch is main

libs/git/commit/traverse.rs

@@ -199,6 +199,16 @@ impl GitDomain {
             return Ok(CommitOid::from_git2(oid));
         }
 
+        // "HEAD" — use repo.head() to get the actual OID
+        if rev == "HEAD" {
+            if let Ok(reference) = self.repo.head() {
+                if let Some(target) = reference.target() {
+                    return Ok(CommitOid::from_git2(target));
+                }
+            }
+            return Err(GitError::InvalidOid("cannot resolve: HEAD (detached or empty)".into()));
+        }
+
         if let Ok(reference) = self.repo.find_reference(rev) {
             if let Some(target) = reference.target() {
                 return Ok(CommitOid::from_git2(target));

libs/service/git/blob.rs

@@ -133,7 +133,7 @@ impl AppService {
         let repo = self
             .utils_find_repo(namespace.clone(), repo_name.clone(), ctx)
             .await?;
-        let rev = query.r#ref.unwrap_or_else(|| "HEAD".to_string());
+        let rev = query.r#ref.unwrap_or_else(|| repo.default_branch.clone());
 
         let tree_oid: git::CommitOid = {
             let rev_clone = rev.clone();

libs/service/git/commit.rs

@@ -853,9 +853,12 @@ impl AppService {
         let repo = self.utils_find_repo(namespace, repo_name, ctx).await?;
         let from_clone = from.clone();
         let to_clone = to.clone();
+        let default_branch = repo.default_branch.clone();
 
         let count = git_spawn!(repo, domain -> {
-            domain.commit_count(from_clone.as_deref(), to_clone.as_deref())
+            let from_ref = from_clone.as_deref();
+            let to_ref = to_clone.as_deref().or(Some(default_branch.as_str()));
+            domain.commit_count(from_ref, to_ref)
         })?;
 
         let response = CommitCountResponse { count };

libs/service/git/contributors.rs

@@ -65,7 +65,7 @@ impl AppService {
         }
 
         let repo_clone = repo.clone();
-        let ref_name_clone = query.ref_name.clone();
+        let ref_name_clone = query.ref_name.clone().or_else(|| Some(repo.default_branch.clone()));
 
         let commits = tokio::task::spawn_blocking(move || {
             let domain = git::GitDomain::from_model(repo_clone)?;

libs/service/git/repo.rs

@@ -70,11 +70,14 @@ pub struct ConfigBoolResponse {
 }
 #[derive(Debug, Clone, Deserialize)]
 pub struct MergeAnalysisQuery {
+    #[serde(default)]
     pub their_oid: String,
 }
 #[derive(Debug, Clone, Deserialize, utoipa::ToSchema)]
 pub struct MergeRefAnalysisQuery {
+    #[serde(default)]
     pub ref_name: String,
+    #[serde(default)]
     pub their_oid: String,
 }
 #[derive(Debug, Clone, Deserialize, utoipa::ToSchema)]