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.

examples/acp-agent/cordis.yml

@@ -82,10 +82,10 @@
     toolName: subagent_fork
 
 
-# Dynamic workflows: the node:vm engine (ctx.workflows) over the spawn subagent
-# backend above, plus the model-facing `workflow` tool. The model writes a
-# JavaScript orchestration script (meta + body); the engine runs it in-process
-# and fans agent() calls out as spawn children.
+# Dynamic workflows: the worker-thread engine (ctx.workflows) over the spawn
+# subagent backend above, plus the model-facing `workflow` tool. The model
+# writes a JavaScript orchestration script (meta + body); the engine runs it
+# in its own worker thread and fans agent() calls out as spawn children.
 - id: workflow-vm
   name: '@deepseek-ai/dsh-workflow-vm'
   config:

examples/coding-agent/cordis.yml

@@ -103,10 +103,10 @@
     toolName: subagent_fork
 
 
-# Dynamic workflows: the node:vm engine (ctx.workflows) over the spawn subagent
-# backend above, plus the model-facing `workflow` tool. The model writes a
-# JavaScript orchestration script (meta + body); the engine runs it in-process
-# and fans agent() calls out as spawn children.
+# Dynamic workflows: the worker-thread engine (ctx.workflows) over the spawn
+# subagent backend above, plus the model-facing `workflow` tool. The model
+# writes a JavaScript orchestration script (meta + body); the engine runs it
+# in its own worker thread and fans agent() calls out as spawn children.
 - id: workflow-vm
   name: '@deepseek-ai/dsh-workflow-vm'
   config: