Add Hindsight integration evaluation and recommendation (docs)

[esafwan]

Motivation

• Provide a thorough architecture-level evaluation of integrating Hindsight as a long-term agent memory system with HUF and recommend a pragmatic integration path.
• Capture comparative analysis, risks, and a concrete Phase-1 implementation plan to guide engineers and product owners.

Description

• Add docs/HINDSIGHT_EVALUATION.md, a comprehensive 400+ line report that studies Hindsight, audits HUF's existing knowledge/memory architecture, compares capabilities, and recommends an integration approach.
• Recommend and detail Option B: embed Hindsight as an opt-in sidecar per agent, including identity mapping, API client hooks (retain/recall/reflect), runtime orchestration points (pre-response recall, post-turn retain, optional reflect), fail-open behavior, telemetry, and UI/agent config fields to add.
• Provide phased rollout plan (two-week Phase-1), architecture sketch, risk register, pilot success criteria, and example pseudocode for integration.
• Document migration/alternative options (knowledge-porting and full-replacement) and rationale for choosing the sidecar approach.

Testing

• This change is documentation-only and did not modify runtime code, so no automated code tests were required or executed for functional verification.
• No unit/integration tests were run as part of this PR because the update only adds a design/evaluation document.

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Codex Task