feat: add resumable historical imports

[shhahhussain]

Summary

• Add CSV, JSON, JSONL, Markdown, text, directory, and provider import paths.
• Add document and atomic-memory lanes with deterministic idempotency.
• Add durable resume, cancellation, bounded concurrency, stale-lock recovery, retries, splitting, and remote ingestion reconciliation.
• Add pilot integration documentation and current Mnemo package branding.

Test plan

• npm run lint
• npm test (14 tests)
• npm run build
• npm audit --omit=dev (0 vulnerabilities)
• CLI plan smoke test
• npm pack --dry-run

Checklist

• Conventional commit title
• Tests added or updated
• Docs updated
• No secrets, API keys, or .env files committed
• No debug logging

This is a CLI package release. No database migration or Mnemo API deployment is required.