refactor(importers): share importer glue helpers

[upsetbit]

Closes #90.

Summary

• add shared importerutil helpers for hashing, raw preservation, RFC3339 parsing, preview truncation, scanner limits, and read-only SQLite opens
• switch Claude Code, Codex, Cursor, Gemini, Antigravity, and Hermes to the shared helpers
• remove duplicated raw-copy helpers from importer packages while keeping traversal/raw preservation coverage

Local validation

• go test ./internal/importers/...
• go test ./internal/importers/... -race
• devbox run -- just lint
• devbox run -- just test-race

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