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_handle.go

@@ -0,0 +1,26 @@
+package infra
+
+import (
+	"bytes"
+	"os/exec"
+	"time"
+)
+
+// ProcessHandle represents a running subprocess with output buffers.
+type ProcessHandle struct {
+	Cmd       *exec.Cmd
+	Pid       int
+	StartTime time.Time
+	Dir       string
+	stdout    *bytes.Buffer
+	stderr    *bytes.Buffer
+}
+
+// ProcessResult contains the outcome of a completed subprocess.
+type ProcessResult struct {
+	ExitCode   int
+	Stdout     string
+	Stderr     string
+	DurationMs int64
+	Killed     bool
+}