feat(knowledge): reinstate lightweight errata generation from blocking (fixes #522)

[mickume]

Summary

• Auto-generate errata when reviewer blocking occurs (critical/major findings stored in DuckDB errata table via migration v19)
• Inject [ERRATA]-prefixed context into coder sessions via FoxKnowledgeProvider.retrieve(), alongside existing [REVIEW] findings
• Persist errata to docs/errata/ markdown files for human visibility

Closes #522

Changes

File	Change
agent_fox/knowledge/errata.py	New module: Erratum dataclass, generate/store/query/format/persist functions
agent_fox/knowledge/migrations.py	Add migration v19 creating errata table
agent_fox/knowledge/fox_provider.py	Extend retrieve() to include errata in coder context
agent_fox/engine/result_handler.py	Generate errata on blocking decisions via _generate_errata()
agent_fox/knowledge/audit.py	Add ERRATA_GENERATED audit event type
tests/unit/knowledge/test_errata.py	23 tests for errata module
tests/unit/engine/test_errata_on_blocking.py	6 tests for blocking + provider integration
6 other test files	Update expected tables/versions for migration v19
docs/memory.md	Document the change

Tests

• tests/unit/knowledge/test_errata.py: Erratum generation, storage, query, formatting, markdown persistence, migration v19 idempotency
• tests/unit/engine/test_errata_on_blocking.py: Errata on blocking, no-errata when not blocking, failure isolation, severity filtering, FoxKnowledgeProvider integration

Verification

• All existing tests pass: ✅ (4292 → 4319 with new tests)
• New tests pass: ✅ (27 tests)
• Linter / formatter: ✅
• No regressions: ✅

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