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/dag_run.go

@@ -0,0 +1,30 @@
+package infra
+
+import "time"
+
+// DagRun represents one execution of a DAG workflow.
+type DagRun struct {
+	ID         string
+	DagName    string
+	DagPath    string
+	Status     string // pending, running, success, failed, cancelled
+	StartedAt  time.Time
+	FinishedAt time.Time
+	Trigger    string // manual, cron, api
+	Error      string
+}
+
+// DagStepResult represents the outcome of one step within a DagRun.
+type DagStepResult struct {
+	ID         string
+	RunID      string
+	StepName   string
+	Status     string // pending, running, success, failed, skipped
+	ExitCode   int
+	Stdout     string
+	Stderr     string
+	StartedAt  time.Time
+	FinishedAt time.Time
+	DurationMs int64
+	Error      string
+}