feat: migrate engine to AgentDB — v2.0.0

[mbocevski]

Summary

Replace raw opslog Store with AgentDB Collection API. Major version bump — no backward compatibility with v1.x task data.

Engine Changes

• defineSchema() with typed fields, autoIncrement IDs, date resolution, 20 virtual filters, computed urgency
• Filter translator — backlog syntax → AgentDB JSON filters (324 lines → 93 lines, 71% reduction)
• Doc storage migrated from filesystem to AgentDB blob API (works with S3)
• Single findAll() in exportTasks — eliminated double read
• Batched doc operations — write/delete use single update instead of 2-3
• Queue drain flag — skip repeat disk reads after first drain
• Redacted error logs — task descriptions no longer leaked

New Files

• src/engine/task-schema.ts — declarative task collection schema

Removed

• Direct @backloghq/opslog dependency (transitive via agentdb)
• Manual validateAttrs() (replaced by schema)
• Manual nextId() counter (replaced by autoIncrement)
• Filesystem docs/ directory (replaced by blob API)

Tests

• 185 tests pass (build + lint clean)
• New: autoIncrement persistence across reopen
• New: doc blob persistence across reopen
• Rewrote filter tests for JSON output assertions

Review Findings Addressed

• Architecture: annotations field documented (AgentDB allows extra fields)
• Performance: single findAll, batched updates, queue drain flag
• Security: redacted task description in error log
• Testing: persistence tests for IDs and docs