feat: add IdStrategy::CommitOid for commit-OID-keyed entity refs

Allows Ledger::create to name refs after the OID of the commit it writes,
providing a permanent, stable identity. Eliminates the need for callers
to bypass create() and manually manage the tree/commit/ref dance.

Closes: #6
Assisted-by: Claude Code (Claude Haiku 4.5)

Author: jdc-pub

crates/git-ledger/src/lib.rs

@@ -26,6 +26,8 @@ pub enum IdStrategy<'a> {
     ContentAddressed(&'a [u8]),
     /// Use the caller's string directly.
     CallerProvided(&'a str),
+    /// Name the record's ref after the OID of the commit that `create` writes.
+    CommitOid,
 }
 
 /// A mutation to apply to a record's fields.
@@ -215,6 +217,28 @@ impl Ledger for Repository {
         fields: &[(&str, &[u8])],
         message: &str,
     ) -> Result<LedgerEntry, Error> {
+        let tree_oid = build_fields_tree(self, fields)?;
+        let tree = self.find_tree(tree_oid)?;
+        let sig = self.signature()?;
+
+        if let IdStrategy::CommitOid = strategy {
+            // Write the commit first (no ref), then name the ref after its OID.
+            let commit_oid = self.commit(None, &sig, &sig, message, &tree, &[])?;
+            let ref_name = if ref_prefix.ends_with('/') {
+                format!("{}{}", ref_prefix, commit_oid)
+            } else {
+                format!("{}/{}", ref_prefix, commit_oid)
+            };
+            self.reference(&ref_name, commit_oid, false, message)?;
+            let fields = read_fields(self, &tree, "")?;
+            return Ok(LedgerEntry {
+                id: commit_oid.to_string(),
+                ref_: ref_name,
+                commit: commit_oid,
+                fields,
+            });
+        }
+
         let id = match strategy {
             IdStrategy::Sequential => {
                 let next = next_sequential_id(self, ref_prefix)?;
@@ -225,6 +249,7 @@ impl Ledger for Repository {
                 oid.to_string()
             }
             IdStrategy::CallerProvided(s) => s.to_string(),
+            IdStrategy::CommitOid => unreachable!(),
         };
 
         let ref_name = if ref_prefix.ends_with('/') {
@@ -241,10 +266,6 @@ impl Ledger for Repository {
             )));
         }
 
-        let tree_oid = build_fields_tree(self, fields)?;
-        let tree = self.find_tree(tree_oid)?;
-        let sig = self.signature()?;
-
         let commit_oid = self.commit(
             Some(&ref_name),
             &sig,

crates/git-ledger/src/tests.rs

@@ -324,3 +324,26 @@ fn delete_nested_field_removes_empty_parent() {
     // Both the nested field and its now-empty parent should be gone
     assert!(updated.fields.is_empty());
 }
+
+#[test]
+fn create_commit_oid() {
+    let (_dir, repo) = init_repo();
+
+    let entry = repo
+        .create(
+            PREFIX,
+            &IdStrategy::CommitOid,
+            &[("title", b"hello"), ("status", b"open")],
+            "create record",
+        )
+        .unwrap();
+
+    // ID should be the commit OID (40 hex chars)
+    assert_eq!(entry.id.len(), 40);
+    assert_eq!(entry.id, entry.commit.to_string());
+    // Ref should contain the commit OID
+    assert_eq!(entry.ref_, format!("{}/{}", PREFIX, entry.id));
+    // Should be readable back
+    let read = repo.read(&entry.ref_).unwrap();
+    assert_eq!(read.fields, entry.fields);
+}