feat(agent): add fail-open capture hooks for raw stream chunks and plan snapshots

[William Lyon (johnymontana)]

Motivation

External tooling (memory systems, telemetry, evaluation harnesses) cannot faithfully
observe an OpenWiki run through the public onEvent contract: the projected
tool_end event carries only {id, name, status} — tool outputs and error bodies are
dropped — subgraph namespaces are collapsed into a main/subgraph flag, and the
agent's explicit planning artifact (openwiki/_plan.md) is deleted on both the success
and error paths before a caller can read it.

This PR adds the narrowest possible seam for lossless observation, without changing
any existing behavior.

What's added

Two optional callbacks on OpenWikiRunOptions (+57 lines across 3 files, no new
dependencies):

• onRawStreamChunk?: (chunk: unknown) => void | Promise<void> — invoked inside
  the stream loop before parseAgentStreamChunk strips fields, so consumers see the
  raw LangGraph [namespace, mode, payload] tuples (tool inputs and outputs/errors,
  subgraph namespaces, message chunks).
• onPlanSnapshot?: (plan: string) => void | Promise<void> — backed by a new
  readTemporaryPlanFile helper; captures _plan.md immediately before cleanup on
  both the error path and the finalize path. chat runs are unaffected (no plan file).

Design constraints

• Fail-open: every hook invocation is wrapped; a throwing consumer only emits a
  debug line ("telemetry and memory capture must never change the OpenWiki run").
• Zero cost when unused: optional chaining; no behavior change for existing callers.
• No reasoning exposure: the hooks surface what the stream already carries; the
  type docs instruct consumers to redact and bound data before persistence.
• Consumers must return quickly (the hook is awaited in the stream loop) — documented
  on the option.

Testing

pnpm typecheck and pnpm build pass; the hooks are exercised end-to-end by
openwiki-graph-reasoning-memory-demo,
which builds Neo4j-backed reasoning memory and an A/B recall evaluation on top of them
(crash-safe journaling, interleaved-subagent correlation, plan-snapshot ordering).

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