Backfill missing git_commit_sha from run branch commit messages

The engine writes git_commit_sha to on-disk checkpoint.json but not to the
metadata branch. When the metadata checkpoint lacks this field, walk the
run branch and match commits by message pattern to fill in the SHAs.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/crates/fabro-workflows/src/cli/rewind.rs

@@ -82,6 +82,10 @@ pub fn parse_target(s: &str) -> Result<RewindTarget> {
 }
 
 /// Build the checkpoint timeline by walking the metadata branch oldest-first.
+///
+/// The metadata branch checkpoint.json may not contain `git_commit_sha` (the engine
+/// only writes it to the on-disk checkpoint). As a fallback, we walk the run branch
+/// and match commits by message pattern `fabro({run_id}): {node_name}`.
 pub fn build_timeline(store: &Store, run_id: &str) -> Result<Vec<TimelineEntry>> {
     let branch = MetadataStore::branch_name(run_id);
     let sig = Signature::now("Fabro", "noreply@fabro.sh")?;
@@ -120,9 +124,66 @@ pub fn build_timeline(store: &Store, run_id: &str) -> Result<Vec<TimelineEntry>>
         });
     }
 
+    // Backfill missing git_commit_sha from run branch commit messages
+    backfill_run_shas(store, run_id, &mut timeline);
+
     Ok(timeline)
 }
 
+/// Walk the run branch and match commits by message pattern to backfill missing SHAs.
+fn backfill_run_shas(store: &Store, run_id: &str, timeline: &mut [TimelineEntry]) {
+    let needs_backfill = timeline.iter().any(|e| e.run_commit_sha.is_none());
+    if !needs_backfill {
+        return;
+    }
+
+    let run_branch = format!("arc/run/{run_id}");
+    let sig = match Signature::now("Fabro", "noreply@fabro.sh") {
+        Ok(s) => s,
+        Err(_) => return,
+    };
+    let bs = BranchStore::new(store, &run_branch, &sig);
+    let run_commits = match bs.log(10_000) {
+        Ok(c) => c,
+        Err(_) => return,
+    };
+
+    // Build a map from node_name to Vec<commit SHA> (newest-first from log)
+    let prefix = format!("fabro({run_id}): ");
+    let mut node_commits: HashMap<String, Vec<String>> = HashMap::new();
+    for commit in &run_commits {
+        if let Some(rest) = commit.message.strip_prefix(&prefix) {
+            // Message format: "fabro({run_id}): {node_name} ({status})"
+            if let Some(node_name) = rest.split_whitespace().next() {
+                node_commits
+                    .entry(node_name.to_string())
+                    .or_default()
+                    .push(commit.oid.to_string());
+            }
+        }
+    }
+
+    // Assign SHAs to timeline entries that are missing them.
+    // For each node, pop from the end (oldest) to match visit order.
+    for (_, shas) in node_commits.iter_mut() {
+        shas.reverse(); // oldest-first
+    }
+    let mut node_indices: HashMap<String, usize> = HashMap::new();
+
+    for entry in timeline.iter_mut() {
+        if entry.run_commit_sha.is_some() {
+            continue;
+        }
+        if let Some(shas) = node_commits.get(&entry.node_name) {
+            let idx = node_indices.entry(entry.node_name.clone()).or_insert(0);
+            if *idx < shas.len() {
+                entry.run_commit_sha = Some(shas[*idx].clone());
+                *idx += 1;
+            }
+        }
+    }
+}
+
 /// Map interior parallel nodes to their fan-out parallel node ID.
 pub fn detect_parallel_interior(graph: &Graph) -> HashMap<String, String> {
     let mut interior_map = HashMap::new();