Change logs dir naming from arc-run-YYYYMMDD-HHMMSS to YYYYMMDD-{run_id}

Embeds the ULID run_id directly in the directory name so logs dirs
can be easily correlated with runs.

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

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

@@ -342,12 +342,13 @@ pub async fn run_command(
     }
 
     // 3. Create logs directory
+    let run_id = ulid::Ulid::new().to_string();
     let logs_dir = args.logs_dir.unwrap_or_else(|| {
         let base = dirs::home_dir()
             .expect("could not determine home directory")
             .join(".arc")
             .join("logs");
-        base.join(format!("arc-run-{}", Local::now().format("%Y%m%d-%H%M%S")))
+        base.join(format!("{}-{}", Local::now().format("%Y%m%d"), run_id))
     });
     tokio::fs::create_dir_all(&logs_dir).await?;
     tokio::fs::write(logs_dir.join("graph.dot"), &source).await?;
@@ -361,8 +362,6 @@ pub async fn run_command(
     // Create progress UI (used for both normal and verbose modes)
     let is_tty = std::io::stderr().is_terminal();
     let progress_ui = Arc::new(Mutex::new(progress::ProgressUI::new(is_tty, args.verbose)));
-
-    let run_id = ulid::Ulid::new().to_string();
     {
         let mut ui = progress_ui.lock().expect("progress lock poisoned");
         ui.show_run_id(&run_id);
@@ -1118,8 +1117,9 @@ async fn run_from_branch(
             .join(".arc")
             .join("logs");
         base.join(format!(
-            "arc-resume-{}",
-            chrono::Local::now().format("%Y%m%d-%H%M%S")
+            "{}-{}",
+            chrono::Local::now().format("%Y%m%d"),
+            run_id
         ))
     });
     tokio::fs::create_dir_all(&logs_dir).await?;

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

@@ -327,7 +327,7 @@ mod tests {
 
         make_run_dir(
             base,
-            "arc-run-20260101-120000",
+            "20260101-ABC123",
             Some(serde_json::json!({
                 "run_id": "abc123",
                 "workflow_name": "my-pipeline",
@@ -366,7 +366,7 @@ mod tests {
 
         make_run_dir(
             base,
-            "arc-run-running",
+            "20260115-RUNNING1",
             Some(serde_json::json!({
                 "run_id": "running-1",
                 "workflow_name": "pipeline-a",
@@ -516,7 +516,7 @@ mod tests {
 
         let dir = make_run_dir(
             base,
-            "arc-run-20250101-120000",
+            "20250101-TOPRUNE",
             Some(serde_json::json!({
                 "run_id": "to-prune",
                 "workflow_name": "old-pipeline",
@@ -552,7 +552,7 @@ mod tests {
 
         let dir = make_run_dir(
             base,
-            "arc-run-20250101-120000",
+            "20250101-TOPRUNE",
             Some(serde_json::json!({
                 "run_id": "to-prune",
                 "workflow_name": "old-pipeline",
@@ -570,7 +570,7 @@ mod tests {
         // Also add a run that should NOT be pruned (too new)
         let keep_dir = make_run_dir(
             base,
-            "arc-run-20260301-120000",
+            "20260301-KEEPTHIS",
             Some(serde_json::json!({
                 "run_id": "keep-this",
                 "workflow_name": "new-pipeline",

docs/reference/logs-directory.mdx

@@ -20,10 +20,10 @@ The log level defaults to `info`. Set `ARC_LOG=debug` or pass `--debug` for verb
 Each `arc run` invocation creates a timestamped directory:
 
 ```
-~/.arc/logs/arc-run-20260307-143022/
+~/.arc/logs/20260307-01JQXYZ123ABC456DEF789/
 ```
 
-The naming format is `arc-run-YYYYMMDD-HHMMSS`. You can override the location with `--logs-dir`.
+The naming format is `YYYYMMDD-{run_id}`, where `run_id` is the ULID assigned to the run. You can override the location with `--logs-dir`.
 
 ### Root-level files
 
@@ -94,7 +94,7 @@ arc ps --filter workflow=my-workflow
 ```
 ~/.arc/logs/
 ├── 2026-03-07.log                    # Daily CLI log
-├── arc-run-20260307-143022/          # One directory per run
+├── 20260307-01JQXYZ123ABC456DEF789/  # One directory per run
 │   ├── manifest.json
 │   ├── graph.dot
 │   ├── run.pid