…ecution

Squashes the workflow engine WIP work onto the latest wolo/workflows
base. The branch had 8 incremental commits; this single commit
captures the cumulative state on top of upstream PR #795 (EdgeBuilder),
PR #796 (NodeConfig), and PR #797 (node name validation).

Architecture:
  * Goroutine-per-node execution model: each scheduled node runs in
    its own goroutine, pushing events into a buffered channel.
  * Single consumer goroutine (runState.run) drains the channel,
    applies state-side effects, yields events to the caller, and
    schedules successors when nodes complete.
  * Replaces the legacy in-process BFS in Workflow.Run with the new
    scheduler, removing findNextNodes and the inline event loop.

New types and constructors:
  * BaseNode (and NewBaseNode) for shared Name/Description/Config
    bookkeeping; FunctionNode and toolNode now embed it.
  * Graph helpers in graph.go: indexed adjacency for O(1) successor
    lookup.
  * RunState (persistable) and runState (in-process scheduler bag)
    in state.go; node lifecycle map (NodeStatus + per-node accumulators).
  * NodeContext wraps InvocationContext with a per-node TriggeredBy
    accessor; agent.InvocationContext gains TriggeredBy() returning
    "" for non-workflow contexts (mocks updated accordingly).
  * Scheduler (scheduler.go): runNode goroutine wrapper, eventQueue,
    cancelAll, and the run consumer loop with sibling cancellation
    and per-node timeout.

Routing and validation:
  * findSuccessors honours unconditional edges, concrete Routes, and
    the Default fallback. Silent dead-ends remain intentional per
    adk-python parity.
  * Workflow.New now returns (*Workflow, error) — picks up the name
    validation introduced upstream by PR #797.

NodeConfig timeout shape:
  * Timeout is time.Duration (not *time.Duration), with zero meaning
    "inherit parent context". Matches net.Dialer.Timeout and the
    http.Server.*Timeout convention; keeps call sites free of pointer
    boilerplate.
  * Adds RerunOnResumeOr / WaitForOutputOr accessor helpers for
    pointer-typed pointer-typed tri-state fields.
  * Adds TestDefaultRetryConfig from upstream PR #796.

Examples and tests:
  * examples/workflow/basic uses NodeConfig{RetryConfig: DefaultRetryConfig()}
    to demo the helper.
  * 7 New(edges) callers updated for the new (*Workflow, error)
    signature.

Tests verified: go build ./..., go vet ./..., go test -race
./workflow/... ./agent/workflowagent/... all pass.