Summarize long channel messages off path

[mostlydev]

Summary

• add cached async LLM message_summary blocks for individual long raw channel messages
• key long-message summaries by source identity/content hash plus provider/model/version
• make /digest prefer message_summary blocks over the full raw excerpt while preserving exact source IDs
• exclude hard events and shorter context-bearing messages from this path
• document CHANNEL_MEMORY_LLM_LONG_MESSAGE_CHARS

Stack

• Stacked on Elide low-value channel acknowledgements #327 / issue-260-low-value-elision
• Elide low-value channel acknowledgements #327 is stacked on Collapse repeated channel decisions deterministically #326 / issue-260-repeated-decision-collapse

Tests

• go test ./examples/channel-memory
• go test ./...

Refs #260