Document multi-graph backfill and submit-without-polling in the skill

[jackaldenryan]

Summary

• Add multi-graph backfill guidance to the building-with-zep skill: submit all episodes to all configured graphs without waiting for processing between graphs or sources, then poll once when retrievability matters.
• Extend the async-ingestion invariant so it applies to every ingestion path (thread.add_messages, graph.add, batch, zep-ingest): submit without polling between adds, poll only on the last episode, and expect wait time to scale with total episode count in the graph.
• Bump plugin version to 0.4.3.

Test plan

• Read updated skills/building-with-zep/SKILL.md and confirm guidance matches intended backfill workflow
• Confirm CI test-plugin.yml passes (manifest validation)

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