feat: add pipeline engine with approval flow and file triggers

Sequential step executor (script, claude_prompt, approval, api_call,
template, skyvern placeholder), reaction-based approvals, file upload
trigger matching, portal API state sync.

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

pipelines/steps/script.py

@@ -0,0 +1,27 @@
+"""Script step — execute a shell command and capture output."""
+
+import asyncio
+import logging
+
+logger = logging.getLogger(__name__)
+
+
+async def execute_script(config: dict, **_kwargs) -> str:
+    """Execute a shell script and return stdout."""
+    script = config.get("script", "")
+    if not script:
+        raise ValueError("Script step requires 'script' field")
+
+    proc = await asyncio.create_subprocess_shell(
+        script,
+        stdout=asyncio.subprocess.PIPE,
+        stderr=asyncio.subprocess.PIPE,
+    )
+    stdout, stderr = await proc.communicate()
+
+    output = stdout.decode("utf-8", errors="replace").strip()
+    if proc.returncode != 0:
+        err = stderr.decode("utf-8", errors="replace").strip()
+        raise RuntimeError(f"Script exited with code {proc.returncode}: {err or output}")
+
+    return output