docs: sync parallel and chat runtime docs

[chubes4]

Summary

• Sync README and workflow operations docs with the shipped default parallel workflow handler.
• Document the Action Scheduler branch executor path and synchronous fallback behavior.
• Sync bridge/channel docs with the shipped fallback agents/chat runtime.
• Update the workflow runner header docblock only, leaving runner code unchanged.

Before/after claim table

Area	Before	After
parallel ownership	Docs described parallel as a product-specific step type not owned by Agents API.	Docs describe parallel as a shipped default step handler.
Workflow built-ins	Docs listed only ability and agent as built-in step types.	Docs list ability, agent, foreach, and parallel.
Parallel concurrency	Runner/header docs described fanout as non-concurrent consumer-owned orchestration.	Docs describe the default fanout contract, the wp_agent_workflow_step_executor seam, Action Scheduler async branch execution when as_enqueue_async_action() exists, and synchronous in-process fallback otherwise.
Action Scheduler branch executor	Docs only mentioned the optional Action Scheduler bridge generally.	Docs mention the shipped branch executor, one async action per branch, branch-specific concurrency gating, loopback runner drain, suspend/resume, and sync fallback without AS.
Chat runtime	Remote bridge docs said Agents API does not ship a chat runtime.	Docs describe the priority-1000 provider-agnostic fallback agents/chat runtime.
Chat runtime configuration	Docs did not state fallback provider/model requirements.	Docs state provider/model resolve from input first, then agent default config, and missing values return validation errors.
Consumer override behavior	Docs implied consumers must provide the chat runtime.	Docs state consumers normally override the fallback with wp_agent_chat_handler at priority 10.

Verification

• Read the listed workflow runner, Action Scheduler branch executor, branch-executor registration, and default chat handler code paths before editing.
• Grepped README/docs for stale parallel and chat-runtime ownership claims.
• Inspected git diff --stat origin/main...HEAD and git log --oneline origin/main..HEAD before opening this PR.

AI assistance

• AI assistance: Yes
• Tool(s): opencode — Claude Fable 5 (Anthropic) orchestrator, GPT 5.5 (OpenAI) implementation subagent
• Used for: Verified code/documentation drift, edited docs/comment text, and prepared this PR description.