Memory Phase 2: Haiku-based fact extraction (Claude backend)

[dcellison]

Folded into #306. See that issue for the current implementation plan; phase-by-phase scope lives in the project's local implementation spec.

[dcellison]

Triage by Kai

Triage summary: Phase 2 of a 4-phase memory system redesign that adds background Haiku-based fact extraction from conversation exchanges, storing structured facts via the existing memory primitive with fire-and-forget async subprocess invocation. Well-scoped with clear acceptance criteria, depends on Phase 1 (#322) being complete first.

Priority: medium
Labels applied: enhancement
Related issues: #322