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_wait.go

@@ -0,0 +1,51 @@
+package infra
+
+import (
+	"time"
+)
+
+// ProcessWait waits for a subprocess to finish and collects its output.
+// If timeoutSec > 0 and the process has not exited by then, ProcessKill is
+// called with graceSec=5 and the result is marked Killed=true.
+func ProcessWait(handle *ProcessHandle, timeoutSec int) (ProcessResult, error) {
+	// Wait for the process in a goroutine.
+	waitCh := make(chan error, 1)
+	go func() {
+		waitCh <- handle.Cmd.Wait()
+	}()
+
+	killed := false
+
+	if timeoutSec > 0 {
+		timer := time.NewTimer(time.Duration(timeoutSec) * time.Second)
+		defer timer.Stop()
+		select {
+		case <-timer.C:
+			// Timeout exceeded — kill the process group.
+			_ = ProcessKill(handle, 5)
+			killed = true
+			<-waitCh
+		case <-waitCh:
+		}
+	} else {
+		<-waitCh
+	}
+
+	// After Wait() returns, buffers are safe to read.
+	exitCode := 0
+	if handle.Cmd.ProcessState != nil {
+		exitCode = handle.Cmd.ProcessState.ExitCode()
+	} else if killed {
+		exitCode = -1
+	}
+
+	duration := time.Since(handle.StartTime)
+
+	return ProcessResult{
+		ExitCode:   exitCode,
+		Stdout:     handle.stdout.String(),
+		Stderr:     handle.stderr.String(),
+		DurationMs: duration.Milliseconds(),
+		Killed:     killed,
+	}, nil
+}