Add [pull_request] support to project config (arc.toml)

Project-level arc.toml can now set [pull_request] with enabled/draft
fields, slotting between cli.toml defaults and workflow.toml overrides.

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

lib/crates/arc-cli/src/init.rs

@@ -39,6 +39,11 @@ root = \"arc/\"
 
 # Disable retrospective analysis after workflow runs:
 # retro = false
+
+# Auto-create pull requests on successful workflow runs:
+# [pull_request]
+# enabled = true
+# draft = true
 ",
     )
     .with_context(|| format!("failed to write {}", arc_toml.display()))?;

lib/crates/arc-workflows/src/cli/project_config.rs

@@ -3,6 +3,8 @@ use std::path::{Path, PathBuf};
 use anyhow::{bail, Context};
 use serde::Deserialize;
 
+use super::run_config::PullRequestConfig;
+
 const CONFIG_FILENAME: &str = "arc.toml";
 
 #[derive(Debug, Deserialize, PartialEq)]
@@ -11,6 +13,7 @@ pub struct ProjectConfig {
     pub version: u32,
     #[serde(default)]
     pub arc: ProjectArcConfig,
+    pub pull_request: Option<PullRequestConfig>,
 }
 
 #[derive(Debug, Deserialize, PartialEq)]
@@ -199,6 +202,7 @@ mod tests {
                     root: ".".to_string(),
                     retro: true,
                 },
+                pull_request: None,
             }
         );
     }
@@ -224,6 +228,20 @@ mod tests {
         );
     }
 
+    #[test]
+    fn parse_pull_request_config() {
+        let config =
+            parse_project_config("version = 1\n\n[pull_request]\nenabled = true\ndraft = false\n")
+                .unwrap();
+        assert_eq!(
+            config.pull_request,
+            Some(PullRequestConfig {
+                enabled: true,
+                draft: false,
+            })
+        );
+    }
+
     #[test]
     fn parse_unknown_field_rejected() {
         let err = parse_project_config("version = 1\nfoo = \"bar\"\n").unwrap_err();
@@ -269,6 +287,7 @@ mod tests {
                 root: "arc/".to_string(),
                 ..Default::default()
             },
+            pull_request: None,
         };
         assert_eq!(
             resolve_arc_root(config_path, &config),
@@ -285,6 +304,7 @@ mod tests {
                 root: ".".to_string(),
                 ..Default::default()
             },
+            pull_request: None,
         };
         assert_eq!(resolve_arc_root(config_path, &config), Path::new("/repo/."));
     }

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

@@ -262,7 +262,7 @@ struct CostAccumulator {
 /// Returns an error if the workflow cannot be read, parsed, validated, or executed.
 pub async fn run_command(
     args: RunArgs,
-    run_defaults: RunDefaults,
+    mut run_defaults: RunDefaults,
     styles: &'static Styles,
     github_app: Option<arc_github::GitHubAppCredentials>,
     git_author: crate::git::GitAuthor,
@@ -278,6 +278,20 @@ pub async fn run_command(
         .ok_or_else(|| anyhow::anyhow!("--workflow is required unless --run-branch is provided"))?;
     let workflow_path = &super::project_config::resolve_workflow_arg(workflow_path)?;
 
+    // Apply project-level config overrides (arc.toml) on top of CLI defaults.
+    // Workflow-level config (workflow.toml) still wins via apply_defaults below.
+    if let Ok(Some((_config_path, project_config))) =
+        super::project_config::discover_project_config(&std::env::current_dir().unwrap_or_default())
+    {
+        if project_config.pull_request.is_some() {
+            tracing::debug!(
+                pull_request = ?project_config.pull_request,
+                "Applying pull_request config from arc.toml"
+            );
+            run_defaults.pull_request = project_config.pull_request;
+        }
+    }
+
     // 0. Load run config if TOML, resolve DOT path, apply defaults
     let (dot_path, run_cfg) = if workflow_path.extension().is_some_and(|ext| ext == "toml") {
         let mut cfg = run_config::load_run_config(workflow_path)?;