feat: add process manager and execution store types (0007b, 0007c)

Process spawn/wait/kill functions for subprocess management with output
capture, timeout, and process group cleanup. DagRun and DagStepResult
types for SQLite execution persistence.

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

functions/infra/process_kill.go

@@ -0,0 +1,42 @@
+package infra
+
+import (
+	"fmt"
+	"syscall"
+	"time"
+)
+
+// ProcessKill sends SIGTERM to the process group of handle, then waits up to
+// graceSec seconds for the process to exit. If it is still alive after the
+// grace period, SIGKILL is sent. Returns an error only if the signal could not
+// be delivered (e.g. the process group does not exist).
+func ProcessKill(handle *ProcessHandle, graceSec int) error {
+	// Send SIGTERM to the process group (negative pid targets the group).
+	if err := syscall.Kill(-handle.Pid, syscall.SIGTERM); err != nil {
+		// ESRCH means the process is already gone — not an error from our view.
+		if err != syscall.ESRCH {
+			return fmt.Errorf("process_kill: sigterm: %w", err)
+		}
+		return nil
+	}
+
+	// Poll until the process exits or the grace period expires.
+	deadline := time.Now().Add(time.Duration(graceSec) * time.Second)
+	for time.Now().Before(deadline) {
+		// Check if process has exited by sending signal 0 (no-op).
+		err := syscall.Kill(-handle.Pid, 0)
+		if err == syscall.ESRCH {
+			// Process group is gone.
+			return nil
+		}
+		time.Sleep(100 * time.Millisecond)
+	}
+
+	// Still alive after grace period — escalate to SIGKILL.
+	if err := syscall.Kill(-handle.Pid, syscall.SIGKILL); err != nil {
+		if err != syscall.ESRCH {
+			return fmt.Errorf("process_kill: sigkill: %w", err)
+		}
+	}
+	return nil
+}