docs: describe the workflow engine as worker-thread first

The outer ring catches up with the engine swap (the package's own
README/JSDoc rode the port commit):

- Seam module doc and README name the worker-thread engine as THE
  implementation, with isolated-vm/separate-process sandboxing as the
  deferred hardening; the seam service doc states the holder-owned-runs
  contract (engine-fiber disposal deliberately leaves live runs to
  their holders).
- Seam contract precision: agentsStarted documents the termination-path
  degradation to the host-observed count; the events section scopes the
  agent-start/agent-end pair to calls that STARTED a child run;
  WorkflowRun wording drops the vm-era abandonment language.
- The dynamic-workflows RFC is rewritten in place to the shipped
  mechanism (implemented-RFC rule): why worker threads, the thread's
  concrete buys, the in-process node:vm first cut recorded under
  alternatives considered; the tool section describes the usage policy
  as the tool's own prompt section.
- gen-doc-graphs: six workflow/* DYNAMIC_EVENT_DISPATCHERS entries (the
  catalog no longer claims nothing dispatches them) and the seam-note
  wording; core-data-structures gains its workflow.md index row;
  packages/README + AGENTS.md layout line + example cordis.yml comments
  say worker-thread; catalogs regenerated.

AGENTS.md

@@ -18,7 +18,7 @@ packages/    Harness packages at packages/<group>/<pkg>/, all named @deepseek-ai
   web/         web seam + search/fetch providers + model-facing web tools
   compact/     compaction seam + basic backend
   subagent/    subagent seam + spawn/fork/ACP backends + delegation tool
-  workflow/    workflow seam + node:vm script engine + the workflow tool
+  workflow/    workflow seam + worker-thread script engine + the workflow tool
   todo/        the todo_write tool
   hooks/       Claude Code / Codex hook bridges + shared wire-protocol library
   session-persistence/  persistence seam + JSONL/SQLite backends