Add tests and docs for recursive run, stage output, error output

Tests:
- `test_run_discovers_dvc_files_recursively`: verifies `dvx run`
  finds .dvc files in subdirs and excludes `.dvc/` directory
- `test_failed_stage_exit_code_and_log`: verifies error output
  includes exit code and creates log file
- `test_summary_file_output`: verifies `$DVX_SUMMARY_FILE` works
- `test_env_vars_are_set`: verifies both env vars are set

README: per-stage commits, error output, `--commit` flag, recursive
discovery.

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

Author: ryan-williams

README.md

@@ -191,7 +191,7 @@ pip install dvx[all]
 ### Running Pipelines
 
 ```bash
-# Run all .dvc computations (parallel by default)
+# Run all .dvc computations (recursive discovery, parallel)
 dvx run
 
 # Run specific target
@@ -205,6 +205,33 @@ dvx run --dry-run
 
 # Force re-run (ignore freshness)
 dvx run --force
+
+# Auto-commit after each stage
+dvx run --commit
+```
+
+#### Per-Stage Commits
+
+Stages can trigger commits by writing to `$DVX_COMMIT_MSG_FILE` (set by DVX before each cmd):
+
+```bash
+# In your stage script:
+echo "Refresh data: 5 new records" > "$DVX_COMMIT_MSG_FILE"
+```
+
+With `dvx run --commit`, stages that don't write a commit message get a default one (e.g. "Run refresh"). DVX also sets `$DVX_SUMMARY_FILE` for short status output.
+
+#### Error Output
+
+When a stage fails, DVX shows the exit code, last 20 lines of stderr, and saves the full log:
+
+```
+  ✗ refresh: failed (exit code 1)
+
+    stderr (last 20 lines):
+      ConnectionError: Failed to fetch https://...
+
+    Full output: tmp/dvx-run-refresh.log
 ```
 
 ### Tracking Data
@@ -309,6 +336,8 @@ with Repo() as repo:
 - Fetch schedules - Periodic re-fetch with daily/hourly/weekly/cron staleness
 - Directory dependencies - Git tree SHA tracking for `git_deps`
 - `dvx import-url --git` - Git-tracked imports with URL provenance
+- Per-stage commits - `$DVX_COMMIT_MSG_FILE` env var + `--commit` flag
+- Detailed error output - Exit code, stderr tail, log file on failure
 - `dvx diff` preprocessing - Pipe through commands before diffing (with `{}` placeholder)
 - `dvx cache path/md5` - Cache introspection
 - `dvx cat` - View cached files directly

tests/test_cli.py

@@ -421,3 +421,37 @@ def test_status_dep_changed(runner, temp_dvc_repo):
     assert output_line.startswith("✗")
     assert "dep changed" in output_line
     assert "input.txt" in output_line
+
+
+def test_run_discovers_dvc_files_recursively(runner, tmp_path):
+    """Test that `dvx run` with no targets finds .dvc files in subdirectories."""
+    os.chdir(tmp_path)
+
+    # Create .dvc files in nested subdirectories
+    sub1 = tmp_path / "sub1"
+    sub1.mkdir()
+    sub2 = tmp_path / "sub1" / "sub2"
+    sub2.mkdir()
+
+    for d, name in [(tmp_path, "top.txt"), (sub1, "mid.txt"), (sub2, "deep.txt")]:
+        dvc_content = {
+            "outs": [{"md5": "", "size": 0, "path": name}],
+            "meta": {"computation": {"cmd": f"echo {name} > {name}"}},
+        }
+        dvc_file = d / f"{name}.dvc"
+        with open(dvc_file, "w") as f:
+            yaml.dump(dvc_content, f)
+
+    # Also create a .dvc/config dir to make sure .dvc/ directory files are excluded
+    dvc_dir = tmp_path / ".dvc"
+    dvc_dir.mkdir()
+    spurious = dvc_dir / "something.dvc"
+    spurious.write_text("should be ignored")
+
+    result = runner.invoke(cli, ["run", "--dry-run"])
+    assert result.exit_code == 0
+
+    # All three .dvc files should be discovered
+    assert "top.txt" in result.output
+    assert "mid.txt" in result.output
+    assert "deep.txt" in result.output

tests/test_executor.py

@@ -296,3 +296,87 @@ def test_run_with_git_deps_in_dvc_file(tmp_workdir):
 
     assert len(results) == 1
     assert results[0].success
+
+
+def test_failed_stage_exit_code_and_log(tmp_workdir):
+    """Test that a failing stage records exit code in reason and writes a log file."""
+    artifact = Artifact(
+        path=str(tmp_workdir / "fail_output.txt"),
+        computation=Computation(cmd="echo fail_msg >&2 && exit 42", deps=[]),
+    )
+
+    output = StringIO()
+    config = ExecutionConfig()
+    executor = ParallelExecutor([artifact], config, output)
+    results = executor.execute()
+
+    assert len(results) == 1
+    result = results[0]
+    assert not result.success
+    assert "fail_msg" in result.reason
+
+    # Log file should exist in tmp/
+    log_path = tmp_workdir / "tmp" / "dvx-run-fail_output.log"
+    assert log_path.exists()
+    log_content = log_path.read_text()
+    assert "fail_msg" in log_content
+
+
+def test_summary_file_output(tmp_workdir):
+    """Test that a stage writing to $DVX_SUMMARY_FILE has its summary shown."""
+    output_path = tmp_workdir / "summary_test.txt"
+
+    cmd = f'echo result > {output_path} && echo "Stage completed successfully" > "$DVX_SUMMARY_FILE"'
+
+    artifact = Artifact(
+        path=str(output_path),
+        computation=Computation(cmd=cmd, deps=[]),
+    )
+
+    output = StringIO()
+    config = ExecutionConfig()
+    executor = ParallelExecutor([artifact], config, output)
+    results = executor.execute()
+
+    assert len(results) == 1
+    assert results[0].success
+
+    log_output = output.getvalue()
+    assert "Stage completed successfully" in log_output
+
+
+def test_env_vars_are_set(tmp_workdir):
+    """Test that $DVX_COMMIT_MSG_FILE and $DVX_SUMMARY_FILE are set to non-empty paths."""
+    output_path = tmp_workdir / "env_test.txt"
+    env_dump = tmp_workdir / "env_dump.txt"
+
+    cmd = (
+        f'echo "COMMIT=$DVX_COMMIT_MSG_FILE" > {env_dump} && '
+        f'echo "SUMMARY=$DVX_SUMMARY_FILE" >> {env_dump} && '
+        f'echo ok > {output_path}'
+    )
+
+    artifact = Artifact(
+        path=str(output_path),
+        computation=Computation(cmd=cmd, deps=[]),
+    )
+
+    output = StringIO()
+    config = ExecutionConfig()
+    executor = ParallelExecutor([artifact], config, output)
+    results = executor.execute()
+
+    assert len(results) == 1
+    assert results[0].success
+
+    env_content = env_dump.read_text()
+    lines = env_content.strip().split("\n")
+    commit_line = [l for l in lines if l.startswith("COMMIT=")][0]
+    summary_line = [l for l in lines if l.startswith("SUMMARY=")][0]
+
+    commit_val = commit_line.split("=", 1)[1]
+    summary_val = summary_line.split("=", 1)[1]
+
+    assert commit_val != "", "DVX_COMMIT_MSG_FILE should be non-empty"
+    assert summary_val != "", "DVX_SUMMARY_FILE should be non-empty"
+    assert commit_val != summary_val, "Commit and summary files should be different paths"