Add base commit and branch to arc run initial output

Shows "Base: main (260a391e5b4c)" after the Worktree line in the run
header, helping users identify the starting point. Uses worktree base
SHA when available, falls back to remote base SHA for remote-only runs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/arc-workflows/src/cli/progress.rs

@@ -695,6 +695,24 @@ impl ProgressUI {
         }
     }
 
+    pub fn show_base_info(&mut self, branch: Option<&str>, sha: &str) {
+        let short_sha = &sha[..sha.len().min(12)];
+        let text = match branch {
+            Some(b) => format!("Base: {b} ({short_sha})"),
+            None => format!("Base: {short_sha}"),
+        };
+        match &self.renderer {
+            ProgressRenderer::Tty(tty) => {
+                let bar = tty.multi.add(ProgressBar::new_spinner());
+                bar.set_style(style_static_dim());
+                bar.finish_with_message(text);
+            }
+            ProgressRenderer::Plain => {
+                eprintln!("    {text}");
+            }
+        }
+    }
+
     // ── Stages ──────────────────────────────────────────────────────────
 
     fn on_stage_started(&mut self, node_id: &str, name: &str, script: Option<&str>) {

crates/arc-workflows/src/cli/run.rs

@@ -550,6 +550,13 @@ pub async fn run_command(
             .show_worktree(wt);
     }
 
+    if let Some(ref sha) = worktree_base_sha {
+        progress_ui
+            .lock()
+            .expect("progress lock poisoned")
+            .show_base_info(detected_base_branch.as_deref(), sha);
+    }
+
     let cwd = std::env::current_dir().unwrap_or_else(|_| PathBuf::from("."));
     let daytona_config = resolve_daytona_config(run_cfg.as_ref(), &run_defaults);
     let exe_config = resolve_exe_config(run_cfg.as_ref(), &run_defaults);
@@ -715,6 +722,18 @@ pub async fn run_command(
         (None, None, None)
     };
 
+    if worktree_base_sha.is_none() {
+        if let Some(ref sha) = remote_base_sha {
+            let branch = detected_base_branch
+                .as_deref()
+                .or(remote_base_branch.as_deref());
+            progress_ui
+                .lock()
+                .expect("progress lock poisoned")
+                .show_base_info(branch, sha);
+        }
+    }
+
     // Create SSH access if requested
     if args.ssh {
         match sandbox.ssh_access_command().await {