Deduplicate ingestion cursor and state management

[BunsDev]

Problem

Cursor tracking logic for ingestion is repeated across import-openclaw.ts, import-cron.ts, and ingestion-state.ts. Each ingestion source implements its own cursor read/write pattern with no shared abstraction. There is also no deduplication logic for events that may be ingested more than once.

Proposed Solution

1. Shared IngestionCursor class — Encapsulates cursor read, advance, and persist operations
2. Event deduplication — Check for existing event IDs before insert (idempotent ingestion)
3. Consistent error handling — Shared retry/skip logic for malformed records
4. Ingestion registry — Central registry of ingestion sources with their cursor states

Acceptance Criteria

• A shared IngestionCursor utility handles all cursor state operations
• import-openclaw.ts and import-cron.ts use the shared utility
• Duplicate events are detected and skipped (idempotent)
• Unit tests cover cursor advancement and deduplication
• No behavioral changes to existing ingestion flows

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