Share single-file importer pipeline

[upsetbit]

Closes #100.

Summary

• add internal/importers/importerutil.RunSingleFile for shared single-file import flow
• switch Claude Code, Codex, Cursor, Gemini, Antigravity, and Hermes JSONL/snapshot import paths to the helper
• keep Hermes state.db bespoke and document the helper in importer architecture docs
• add focused helper tests for hash skip, normal write, and parsed-ID no_usage behavior

Local validation

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

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