feat(import): add memory draft import

[Grivn]

Summary

Adds a versioned memory draft schema and mnemon import command for converting historical chat exports into Mnemon memories.

Details

• Defines schema_version: "1" draft structures with validation and source resolution.
• Imports draft insights through Mnemon's normal write behavior for deduplication, embeddings, graph edges, lifecycle scoring, and pruning.
• Disables real-time temporal edge creation during historical import, then repairs affected source timelines after all imported insights land.
• Refreshes effective importance after explicit edges and temporal repair before auto-prune runs.
• Documents the import schema, field constraints, category/importance guidance, and a copy-paste LLM extraction prompt.
• Adds tests for draft validation, backdated temporal repair, and EI refresh after explicit edges.

Validation

• go test ./...
• go build -o mnemon .
• git diff --check

Refs #32